This is only an incomplete listing which I’ll go back to later and add to (maybe), but please feel free to suggest anything I’ve missed and I’ll add it here.
 
D&D: arcane (wizard, sorcerer), divine (cleric, druid, bard (depending on edition, may share spell lists with druids/wizards).
2E has psionics, wild magic, rune magic; 3E has binders (entities inhabit caster to give them special powers depending on entity), truename magic, shadowcasting, elemental magic (shukenja), an oriental wizard variant (wu jen), warlocks. Assassins, demologists, blighters, and a few others other specialized characters have unique spell lists; however most types of magic are fairly similar. FR Magic of Faerun IIRC includes gem magic, gnome artificers. Eberron includes an ‘Artificer’ class.
 
Palladium Fantasy: Wizardry, Psionics (Mind Mage), Diabolism (rune magic), warlockry (elemental), witchcraft, Alchemy, healing; priests gain limited wizard spells+ unique special abilities. In supplements- Necromancy, Shamanism.
+Rifts includes Techno-wizardry, Tattoo Magic, the bio-magic of the Splugorth, gem magic, temporal magic, biomancy (hippie nature magic), mystic smithing, nature magic, fire sorcery, whale spellsingers & ‘Koral’ shaping.
+Mystic China: Chi magic, mudras
 
LegendQuest: druidic, healing, illusion, necromancy, sorcery, spellsinging, alchemy, enchantment, mentalism.
 
Talislanta: Biomancy, Cryptomancy, Erythrian Battle Magic, Invocation, Mysticism, Natural Magic, Necromancy, Pyromancy, Ritual Invocation, Sorcery, Thaumaturgy, Witchcraft, Wizardry
 
Dragon Warriors: sorcery, mysticism (psionics), elementalist (includes evil “Darkness” elementalists), warlock (battle-mage/fighter)
 
Shadowrun: hermetic, shamanic, technomancy
 
Mage: the Ascension has several ‘flavours’ of magic, but with all sharing the same basic magic rules. Types include the Technocracy, virtual adepts, order of Hermes, Euthanatos, etc.
 
Ars Magica: Hermetic, Hedge and Witchcraft
 
 
Fiction:
Interesting fictional examples of worlds with multiple magic systems I can think of:
 
 Master of the Five Magics: Alchemy, Sorcery (mind control), Wizardry (demons), (item creation), Thaumaturgy, Magic (item creation)
 
The Misenchanted Sword series: sorcery (technology?), wizardry, witchcraft (psionic?), demonology, warlockry (telekinetic), theurgy (divine magic).
 
Jack Vance’s short story “Green Magic” has colour coded magicks (White, Black, Purple, Green).

This is probably the last post on magic, though its entirely possible I've missed major subtopics. Apologies that this one is a bit random.
 
 
Power limits: maximum spell level/spell point cost of spell useable is typically limited by a character number gauging their spellcasting power; this might be the characters level, an attribute score (s), or a specific skill score. Spell levels may be equal to character level, or interspersed; for D&D wizards there are only 9 spell levels (across 20+ character levels) - with the memorization system giving a character a new spell level each level would also mean a new spell slot, and so an inordinate number of spells.
3E D&D has 'metamagic feats' which let a character adjust the level of a spell to add some extra effect, while 5E D&D lets characters expend higher-level spell slots for e.g. more damage.
 
Spells with levels sometimes have a 'magic point' cost connected to the spell level, while in other systems (Savage Worlds, Tunnels and Trolls) spell 'difficulty' and 'energy cost' are not connected to each other; games like this usually have 'spell points' growing much more slowly with level, since increases aren't automatically necessary to use higher-level spells; levelling up may instead unlock spells which use power more efficiently (lower point costs).
 Damage is usually connected to spell level in some way. SenZar has a set conversion rate between power pool points/damage, with level only increasing the damage cap, while in Talislanta 4E (IIRC) spells have level-based damage with the caster determining level as the spell is cast (this also modifies the success chance of casting, with the unfortunate side effect a higher level spell is less likely to 'critical' and so could do less damage than a low-level spell despite the higher cost).
Most other systems determine damage/effect on a spell-by-spell basis (Spell A does X base damage, Spell B does Y base damage). Ability modifiers may apply to damage (for example, for 4E D&D wizards who get +Int to damage for most spells). 'Effect-based (in the HERO sense) magic systems can have spells that deal damage based off a power rating assigned, which is not particularly compatible with the idea of 'spell levels'. Spells may also automatically scale up based off caster level/magic skill; the simplest case being D&D here (up to say 3E like fireball doing d6/level; 5E's approach where a fireball is fixed 8s6 unless you spend a spell slot being a change to more resource-based). As noted previously, LegendQuest has an interesting system where a magic-user distributes 'control levels' (equal to their skill with a given spell) across various aspects like damage, range, etc.

Games often let characters spend extra resources to pump the power of spells. D&D 3E has 'metamagic feats', D&D 2E has additional spells that sometimes augment other spells (e.g. the 'Vocalize' spell in Complete Wizard lets a wizard cast another spell silently, while 'Squaring the circle' in Tome of Magic increases a spell's range). SenZar lets characters spend extra power to boost area of effect, while Savage Worlds lets a character spend extra power points on bolt/blast to increase the damage. Synnibarr lets multiple characters pool their power to cast stronger spells and has a 'Winds of Enforcement' spell that lets wizards of high enough level multiply the effect and drain of a spell, with chance of backlash if too much power is used.

Implements/aids to magic: 4E D&D gave wizards staves which, like enchanted weapons for fighters, add ‘plusses’ to the wizards’ powers (staff +3 = +3 to attack rolls and damage with a wizard spell, assuming this spell has the ‘implement’ keyword). (Not in itself a bad change, IMHO, though a radical departure from prior editions where staves contained a number of charges of bonus spells)
Tunnels and Trolls allows wizards to have magic staves which subtract the wizards’ level from the Strength cost to cast a spell; the normal ‘ordinaire’ staff is slightly expensive (100 GP), although a wizard can use a L1 spell to construct a ‘makeshift staff’, which would possibly explode when first used (1st level save on Luck to not explode) and eventually burn out (after a given amount of use). It also had ‘deluxe staves’ – demons bound into staff form - which were semi-sentient and indestructible, learning any spells cast through them. A spellcasting aid could also take the form of a ring, enchanted jewel, etc.
Dragon Warriors lets sorcerors create a staff, but at a cost of their own permanent spell points; 2 spell points lost forever become 3 in the staff, which can be used only for the staffs particular sublist of spells (depending on its theme). Staves were effectively indestructible, but losing them was a major hazard to the owner (reminiscent of Sauron and his magic ring).
 
Power components: extra ingredients might add extra effects/power to a spell, or absorb some of the cost.



Magic Resistance aka Spell resistance: games frequently have magic resistance as a monster quality. Tactically, in terms of how much the wizard is boned, this parallels weapon immunity or damage reduction to the fighter types, or perhaps heavy armour. While armour more commonly (across game systems in general) reduces damage, magic resistance tends to negate or bounce a spell completely rather than reduce its effect, presumably since in most systems it is difficult to adjudicate what happens with a reduced effect spell (much like when a rings of spell turning makes a target take 60% of a charm person...). D&D golems are interesting in that the original D&D golem is probably immune to magic as a lazy shortcut to listing lots of specific immunities due to its material and lack of mind or metabolism; by 3E the golem had 'magic immunity' as a canonical feature even though all the specific immunities granted by the construct type, mindless condition, etc. and explicitly defined.
If an “armour bypass roll" system is in use for armour, then magic may function analogously; or if the game is e.g. a dice-pool system and the effects of 1 success, 2 successes, 3 successes etc. on a spellcasting roll are determinable, magic resistance might be partial; as an example I will mention Arkham Horror here although its a boardgame rather than an RPG – here characters get bonus dice on Fight rolls (successes equal damage) for either weapons or spells, with resistances halving the bonus dice and Immunity negating it – for example a monster with Magical Immunity ignores a spell, while a monster with Physical Resistance takes only ½ the bonus dice from weapons – the character’s base combat dice still apply.
In other approaches I vaguely recall seeing a magic system where MR instead functioned by reducing spell duration, so a spell was thrown off more quickly, and there are specific cases (e.g. some anti-psionic feats in 3E) where using an ability costs more points vs. a resistant target. Valley of the Pharaohs contained a monster from Egyptian mythology, which was difficult to affect with magic because spells attacking it required speaking its true name, which was inordinately long and thus increased the casting time of spells).
As well as a monster/racial ability, magic resistance may also be gained from class (the Nega-Psychic in Beyond the Supernatural?) or advantages (e.g. Savage Worlds' Arcane Resistance Edge, giving +2 on opposed rolls and +2 Toughness vs. magical damage).

Also, some misc things (quoting from Ron Edwards' Fantasy Heartbreakers essay)

Quote from: HeartbreakersHahlmabrea: A character learns several spells in a given category, but may cast them in any combination for synergistic effects. Improvising novel combinations and outcomes is highly encouraged and supported by many examples in the text. The best thing about this system is that combining spells actually reduces the chance for Spell Error, rather than increasing it - creating incentive to combine and be creative.
Legendary Lives: A character knows a category or two of magic, with no further compartmentalization - no individual spells. Instead, the player may customize applications of the category on the fly during play. So if you know "Fire" spells, anything and everything about or using fire is yours to do. It's limited only by "cost" (0-5) as set completely freely by the GM, which is the one sour note in the mix, as it sounds to me like a recipe for bitter argument. (LL's system is similar to than Ars Magica's, but more sketchy and flexible.)...
...Forge: Out of Chaos: The player picks a character's spells, but each one is heavily randomized for many aspects, including points to cast it, degree of side effects, distance, duration, and other things. One begins with an allotment of re-rolls for these aspects of the spells, and increasing the character's level includes more re-rolls, permitting the player either to add new spells or refine the old ones as desired.
Dawnfire: First of all, every character is a magic character in this setting, with Flow Points and access to a variety of magically-oriented abilities; "magic users" are simply those who have optimized the same abilities that everyone has. Second, the game includes the universally-accessible mechanic called bullshitting, which is just about the neatest magical mechanic I've seen in print. To "bullshit" is to cast any spell in the book or that the player can make up, regardless of what is or isn't written on the character's sheet. Anyone can try it, any time; having a certain skill makes it a little safer, but that skill is not necessary.

Dawnfire also deserves some special mention for its encouragement to customize the magically-oriented characters as desired, such that one character might be a singing bard-type or whatever ... this is pretty standard, until I got to the suggestion regarding "na?ve" magic-guys - who must bullshit all their spells and never realize that they are casting magic. The player must describe all the outcomes of their spells accordingly, manipulating elements of the environment to suit.

Good and Bad Power Writeups
A few spells (or similar things) are badly written, largely due to interaction with other rules, that they more or less don't work RAW. A particular problem is when a spell's casting time offsets its actual use.
*e.g. initiative issues - 2E D&D 'energy containment' is a defensive power which basically negates incoming energy attacks. However, it doesn't have a duration and can't be maintained, so probably requires successfully guessing what the monsters are doing, and declaration of action use, before initiative - I'm not 100% sure. (by comparison Star Wars D6 [2E] absorb/dissipate energy clearly does require an action, but lasts for a round afterwards).
*in 3E D&D, Xephs (a psionic race) have a supernatural power which increases their movement speed, but there's no indication this has an unusual casting time, so by RAW it takes a standard action to use and thus limits the xeph to a single move. The power lasts about 3 rounds so it could be useful, but is limited to edge case use by dodgy design, probably unintentionally.

*conversely in possibly good, or at least interesting, design, 4E D&D has healing powers like Healing Word operate as single target close bursts ("one ally in burst") rather than being ranged attacks; consequently these powers don't trigger opportunity attacks as ranged attacks do (discussion here). Slightly similar maybe 3E Faerie Fire is a burst which affects a 5-ft radius as its intended to affect invisible creatures and would be stopped by the rules prohibiting targeting vs. full concealment if it was a ranged attack.

Keywords
Keywords are an idea that I believe was particularly popular with CCGs, that sort of made the jump to RPGs; they start appearing e.g. in 3E D&D spell descriptions (tags like [Fire], or [Mind affecting]) and can be useful. In 4E these were used more often, sometimes initially not doing much but with the idea that later feats/powers would combo off them.

Some games use an informal keyword approach where a word that's part of the name of the power, or for some card games any text on the card, becomes a keyword. For example, from Feng Shui, Integration of the Clouds (fu power) lets PCs 'combine use of two fu powers in one action; affected powers must share a word in their titles (such as 'Fox' or 'Fire'. Fu powers with contradictory conditions may not be combined. Pay chi cost for both, but only the highest shot [i.e. action point/initiative delay] cost. While a fairly cool trick, this obviously adds a need for oversight on power titles (which would otherwise be just fluff text), which rapidly becomes problematic as # powers increase. Its also largely an arbitrary restriction unless the writer has taken pains when naming their powers, might generates a conflict between clarity in power naming and banning combinations (Pro-X and anti-X powers both include X), and - in this case - brings up a question of whether the same power can be activated twice. May generate interesting linguistic problems in translation, or homophone issues e.g. if 'wind' (air) and 'wind' (watch) are the same word.

Naming Conventions
Spells are often named fancifully or after its alleged creator (one D&D spell, Nystul's Magic Aura, is instead named after the player). While fun extensive grimoire's have the potential to be annoying where indexed alphabetically since e.g. "Sharthyd's Spell of Evil Detection" is under S, rather than D for Detect Evil. Or may be a mix of that and things like "Speed up Spell, Jon's" depending on exact name structure (Compleat Arduin).

Next - monsters and NPCs.

“The Gibbelins eat, as is well known, nothing less good than man. Their evil tower is joined to Terra Cognita, to the lands we know, by a bridge. Their hoard is beyond reason; avarice has no use for it; they have a separate cellar for emeralds and a separate cellar for sapphires; they have filled a hole with gold and dig it up when they need it. And the only use that is known for their ridiculous wealth is to attract to their larder a continual supply of food.”
-The Hoard of the Gibbelins; Dunsany.
 
OK, monsters.
Other game systems interrelating to the monster rules include: hit location, hit points, Size, special abilities/powers, attacks per round, attribute scores, races, movement/miniatures, NPCs.
Weapon rules also often interact regarding natural weapons. A monster might have a single damage range representing multiple weapons (opening up more options for the DM describing attacks) or bite/claw/tail slap might have distinct damage ranges and attack bonuses. The latter can sometimes turn into the monster spamming the heaviest damage attack over and over [SenZar], or lots of rolls as each item is checked separately e.g. a 3E dragon has five physical attacks normally -2 claws, a bite and 2 wing slaps - as well as distinct rules for other options such as tail slaps or crushes. D&D4 monsters alternate attack bits by switching between powers with various cooldown times. (Another way to represent multiple natural attacks might be to add a hit bonus for extra bits, then roll between limbs if they have different damages).

 
Some interesting systems/ideas with regard to monsters:
 
*Tunnels and Trolls has single-stat monsters – very quick to create; a creature’s “Monster Rating” determines both its combat roll (dice+adds) in combat and its hit points. This works largely because T&Ts combat system is quite simple, with all attacks simultaneous and damage equal to difference between both sides attacks. There are guidelines for converting MR back into attributes where necessary e.g. using the score as CON and distributing adds among the stats that give a combat bonus in T&T: Str, Dex and Luck. This is something of a 'kludge', but ends up seeing use when e.g. a monster has to use a ranged weapon (DEX check) or other save. Particular problems occur when a MR-rated creature is used as a PC (e.g. one solo turns a character into a tiger with MR 186).
The MR system also makes stats for conjured creatures very simple (often MR is based off a couple of the wizards's stats added together). Another interesting use of MR is in one solo adventure ('Soul Survivor') where a PC adds together their WIZ and CHR to get an MR to be used in a psychic battle, using normal MR rules (an interesting idea which could be the basis for any sort of weird 'skill challenge' situation perhaps - from CHA+IQ diploma battles to STR+CON pie-eating contests).
T&T sometimes has two-stat monsters with a 'Fighting MR' and a 'Constitutional MR'. (I'd toyed with idea that some other checks could use Con-MR as well e.g. a centaur might have a large Con-MR due to body mass, and use that for STR checks with its horse half too).
Partly T&T derived free rpg Twisting Tunnels (see link in 'multidie additive' post) has a similar MR system, but possibly more useful because its core ability check system is compatible with the "MR" dice, instead of MR being largely on a different scale to other stats that requires conversion- working in a freeform fashion similar to how e.g. 'Over The Edge' traits work:

QuoteA monster’s Hit Dice can be used for anything that makes sense: A dull-witted troll might use its Hit Dice for smashing or devouring hapless delvers, but not for a chess tournament. A cunning sorceress might use her HD for dueling wizards or beguiling warriors, but not for a pie-eating Clash.

(as an aside, I once was fiddling around with an MR-based system for treasure generation, where the GM could use 'effect' from an MR roll to generate not just damage but also how much treasure its carrying and/or what magic items it has - # successes being converted into a + or level of magic item (rolling separately for gold and magic). The main issue I had with this was that inflation in MR didn't give a steep enough gain in magic item power - the dragons didn't have stuff that was much cooler than the orcs did. Potentially fixable, though).


In other more modern games, I suspect the One Roll Engine probably isn’t far behind T&T Monster Rating in complexity since one roll determines hit, damage, hit location and initiative, although at the least a creatures’s hit points are going to be separate in ORE.
Less extremely, Atlantis (Omega system) gives monsters an 'adversary level' which replaces separate Combat Rating [attribute] and skill rating- a monster applies its full level to either attack or defense, then only 1/2 to the other option.
 
*Feng Shui included “mooks” or “minions”, bad guys who died after a single blow, an idea later also in 4E D&D; this did make these guys follow different rules to PCs as regards hit points (e.g. in 4E everyone except them added their Con to HP), and sometimes raises questions like “how did this dude get to 30th level with 1 HP?” or weirdness with the larger monsters (1 HP ogres). In 4E, this is usually related to the idea of monsters having multiple stat blocks; an orc encountered at level 10 might be the 'same' orc as at level 1, with its HP and defense scores however switched out to make it harder to hit (level appropriate) but easier to kill (minion). In a sense this is part of the 4E concept that stats exist only 'on camera' for dealing with PCs, as opposed to the system functioning as a 'physics engine' where the GM can extrapolate rules interactions for worldbuilding.

Minions do however ease tracking on the GM, for large combats. AD&D implemented this effect more or less by having creatures varying from 1 HD to lots of HD, depending on size or level; however, an equivalent effect can also be set up where HPs start out relatively even (i.e. =CON for everybody) by having level differences (e.g. amount an attack roll exceeds defense, or special attacks) significantly adding to damage.
Savage Worlds’ version of this was interesting; most creatures have only 1 Wound, but a damage roll must beat a target’s Toughness to damage it; each +4 rolled over toughness generates 1 “wound”, with just beating it resulting in a target instead being “shaken” ( a second shaken result gives a wound). The system works well for mass combat, but perhaps not as well for combats against single, larger monsters; combat against one may include a few rounds of nothing happening as characters try to get lucky, unless the players are clever enough to work together with tricks/stunts (or get lucky and explode it on round 1).
(Note on getting around HP tracking: its been suggested that lots of HPs can also be tracked by having a "pile" of HPs, equal to [monster HP x number of monsters] and just having monsters die each time points equal to the monster HP are lost. This assumes PCs will focus fire - which they probably would try to do anyway.)
 
*AD&D had monsters without ability scores, which generally worked (they did have Int, and occasionally would require other scores - these could be rolled up or GM assigned if necessary, with Strength probably being the most-needed stat for various combat maneuvers). This generally worked since with or without rolling a statistic, as the monster's capabilities would be largely the same - the AD&D bonus charts gave very few modifiers in the 9-15 range. PCs with very high stats could get unfair advantages over creatures, who had no Con bonuses to their Hit Dice or (usually) Str modifiers for damage. Giant-type monsters in 1E had fairly arbitrary damage but Large weapons were standardized (double normal dice i.e. longsword 2d8) and Str modifiers added in 2E.
Basic occasionally gave larger monsters a + to damage (e.g. orc chieftain - +2 to damage), representing Str.
3E’s universal system applied modifiers to most tasks, and mandated inclusion of attributes - which were often quite high and generated huge modifiers; this made monster creation more complex and made monsters of the same HD/CR vary in challenge considerably (albeit realistically).
4E kept monster statistics, but by fiat disjoined them from the system by declared that monster attributes did not modify their attacks, defences or hit points, in order to keep these precisely level-appropriate. (edit 27/4/2014: to be fair, looking closely at this now it seems that the numbers aren't far off - possibly the designers set monster ability scores after setting values with the role/level table, giving them appropriate numbers in one of each defense pair to get nearly-correct final defense values).
Palladium perhaps also bears mentioning: this has rolls for monster attributes, with this often being useless or unnecessary since there are no attribute checks and combat modifiers start only at very high values.

*3.x D&D also had a complex and somewhat confusing system of monster “types” (undead, fey, animal, magical beast, etc) though these were poorly defined and sometimes overlapping; types could be based conceptually off size (giant), monstrous features (aberration, monstrous humanoid), shape (humanoid, ooze), composition (elemental), or origin (outsider). The types made adjudication of effects such as whether a given spell would work easier but also functioned as classes for monsters, overcomplicating new monster design by setting HD, BAB, and skills. Low-Int but very big monsters would get excessive skill points.
 
*Runequest has one of the best (IMHO) systems for representing monster statistics; it includes both detailed hit location tables specific to each monsters, other monster attributes, and gives monsters Size scores (this automatically feeds into determining hit points, instead of being fairly ad hoc like most games). The system lets monsters have varying armour by location (something D&D tried and struggled with in older editions before giving up), and hit locations let PCs do things like cut the legs off the giant spider etc. Runequest is also noteworthy for having “incomplete creatures” (things without some ability scores, e.g. incorporeal monsters with no Str scores, or soulless monsters with no Power stat), an idea Tweet later adapted into 3E D&D (“nonabilities”). In many cases 'nonabilities' are indistinguishable from scores of 0, although they bypass 'death at zero attribute' rules and prevent attacks against the weak ability (making ghosts ungrappleable for instance).
RuneQuest monsters have stats and skills in the same fashion as do PCs, and can be used as characters directly (although some might make especially powerful PCs).
 
*The Atomik Alienz supplement for Fuzion had an interesting set of choices for SF monster design including circulation (open, closed, osmotic, raw energy, none); type of blood (warm, cold, hydrocarbons, gaseous); composition (silicate, carbon-based, robotic, pure energy), respiratory system (absorption, gills, no respiration), nervous system (electrochemical, semi/superconductor), locomotion method (aquajets, slither, monopod), etc etc. From a biology perspective it was fascinationg though the detail generated was largely a curiosity – the mechanics for Fuzion did not particularly make these details important. I’d have loved to see this written for a game with bleeding rules, detailed hit location (got your spleen!!) and so on.
Things like redundant internal organs or lack of vital organs can also be represented fairly well in games with multiple sorts of HPs (Palladiums Hit Points/SDC, or the Alternity set of three separate types, Mortal/Wound/Stun).
 
*GURPS Lensmen as noted earlier has some really weirdly shaped aliens, something that’s interesting with its minis rules. (GURPS, or any point-based powers system, is probably going to be good for putting together new monsters, though the process may or may not be easy).
 
*Systems may have precise ways of working out how tough a monster encounter will be (“encounter level”), or even prescribing how difficult an encounter should be. The most notable example of the latter may be 3:16 (the High-Ronny-Award Winning Starship Troopers-ish game); this gives the GM a limited number of “Threat Tokens” to use per planet. Successful Fighting rolls let PCs remove Threat Tokens, with each “Threat Token” representing a variable number of aliens, probably lots.
 
4E has a set of statblocks for monster roles (Ambusher, Lurker, Artillery, etc.) at various levels, which are the basis for designing new monsters - one oddity here being that monsters seem to sometimes change types i.e. the younger version of Green Dragon is an ambusher while the older version becomes a controller. Fantasy Craft reportedly uses a numerical system where monsters are rated from 1 in 10 (in roman numerals) for various traits; that value is used with the monster's level to determine its final bonuses in a category, so that it can be threoretically scaled up with level differently in each aspect.

AD&D has only vague CR guidelines. It is interesting in that some monsters require magic weapons to hurt, which has the effect of limiting those to powerful characters indirectly (and probably prevents henchmen or hirelings from helping out unless they can think of something clever to do).

Monsters may sometimes be fit poorly by a 'humanocentric' attribute scale and rules. In particular a single score for e.g. Strength may not represent well creatures which may be very strong in some muscle groups/circumstances and weak in others, like centaurs (upper body strength much less than lower half, and expect chin-ups or climbing to be impossible) or cave fishers (the D&D cave fisher has an 18/00 Strength for the filament it uses to 'fish', but no damage modifier to its pincers; the pit fiend is exactly the opposite, with an 18/00 Strength for melee damage while its tail constrict can be escaped with an unmodified Strength check by the victim. Interestingly if AD&D had used attributes, it would probably end up shoehorned into using the same Str for both). # legs sometimes figures into Balance checks (e.g. 3.5 vs. Trip attacks).
 
In systems that aren't too complex or 'simulational', monsters are often reskinned as other types (many AD&D monsters have notes like 'treat female carnivorous apes as gorillas for HD and attacks, but allow them the same keen senses as the male' (Dragon #133), or 'treat young storm giants like ogres'. This sort of thing can also be handled to an extent by monster advancement (reduction) rules, or even rules for varying a monsters' "ability scores".

Below: from Different Worlds magazine #35, the Star-Devourer is a particularly sexy Lovecraftian worm monster in Superworld (APP -Appearance- of 50). Note the complexity of the stat blocks - in T&T the GM could just assign "Monster Rating: 1000" and be done with it.


Monsters as pets: [section to be expanded]
Link discussing problems with 5E ranger here ; tl;dr - ranger companions are selectable based on CR which is partially based on HD, but actually get their own minimum HPs, throwing off many but not all CR estimates. Generally can't benefit from ranger buffs (hunter's mark), but may be able to use own special abilities. Specific problem with Multiattack.

Miscellaneous: HarnMaster shares illustrations between similar monsters, such that the GM may show the PCs the monster illustration from the book without definitively tipping off the players whether they're fighting e.g. a sentient undead (Ahmorvus)(sp?) or just a zombie.

Link on monsters as challenge vs. monsters as simulation on rpg.net. Winning quote here:

Quoteit seems to me that most of the time they started with a goal, and then worked backwards to justify it, so it didn't feel it was really simulating anything in particular either. Natural armor was especially bad because it felt to me like it usually based on the creature's cr rather than how tough its skin looked.

Although, IMHO, it is useful to differentiate some aspects - dodging from armour - to be able to say whether armour can be looted or immobilization will stop a monster, it is possible to go too far.

Attacks per round: how many attacks a monster gets depends a lot on how common multiple actions is in the system, as well as whether a monster is normally designed as a single boss that fights several PCs or as one among a horde. Usually monsters get as many attacks as PCs (or more, for a boss-type monster). D&D is an interesting case in that 4E 'solo' or 5E 'legendary' creatures explicitly get extra actions. In AD&D only fighters got extra attacks with level, so few monsters do; 3E generally kept monster attacks the same as that, despite all PCs now getting extra attacks (based on base attack bonus); the exception being weapon-using monsters which got extra attacks with level; a fire giant (for example) went from 1 attack to 3 due to their Hit Dice, making it more dangerous since its size gave it both huge damage and multiple attacks due to its HD.
Other monsters had multiple attacks due to multiple natural weapons - dragons, krakens, whatever. Or conversely to the fire giant there are epic-level (i.e. over 20th level) monsters that get only 1 or 2 attacks. A hunefer (god mummy) for example gets 2 slams despite a +25 base attack (50 HD), which would entitle it to 4 iterative attacks with a weapon (like a PC); the monster description tries to give them 'unbelievable celerity' by adding Haste as a spell-like ability.

Conceptual note - note that the separate approach for monsters and PCs in 4E, has a slightly peculiar effect that many almost-PC creatures like bandits had monster stat blocks. In 2E or 3E by contrast, something being a monster implied that it was a specific monster race; that could make it raidable by players, using monster-as-PC race creation rules. But its interesting that say something like a 'hag' in 2E (your traditional witch) is weird in being a monster type, while the similar statblock in 4E could have a monster writeup and yet still be just an old lady.

Monster Treasure: monsters in AD&D got a numerical code which tended to generate specific things (gold, gems, potions, weapons, etc)(sometimes weirdly specifically). A bit more abstractly, Dragon Warriors just gave each a rating from 'scant' to 'bountiful' that scaled up fairly non-specifically.

Random generator online:
https://www.cobwebbedforest.co.uk/workshop/Treasure
(the above is a bit awkward in that it has separate rolls for is there, then how much; for some of these it could be made one roll if there were e.g. d12-2 items or so on). The treasure generation system here is also an example of an 'effect' system, a bit like working out damage after a 'hit' determines there's treasure. In other games this may be more GM-determined.

3E gave monsters a challenge-rating appropriate amount of treasure, which the GM could assign although the amount was reverse engineered from the random treasure generator by level; some monsters had no treasure which others to compensate got double.
(the system didn't balance characters who were levelling by killing more lower-level monsters, which could generate more treasure).
4E was even stricter in how many 'treasure parcels' were found per level.

NPCs are in most systems similar to PCs. By this posts nature, it concerns itself primarily with systems where NPCs are treated differently to PCs, although my personal preference is generally for them to be treated as similarly to PCs as possible.
Systems may do it to ease tracking that would otherwise be too complicated, because of actual assumed differences between PCs and NPCs (e.g. PCs being assumed to be adventurers/heroes and hence exceptional). Systems may treat NPCs as “narrative constructs” rather than fictitious people in a fictitious world - this could include setting PC hit points at a level that makes them readily fall over dramatically (minions), or setting DCs to influence an NPCs at a level-appropriate rating for the PCs.
An extreme case of this would be Apocalypse World, where NPCs have no statistics; they have a health (less than a PC) and may inflict damage (based on weapon), but all rolling is done by PCs, with NPC ability not giving any adjustment. Dungeon World is similar: notably, PCs can get "instant kill" results on NPCs due to rolling for Hack & Slash against lower-level opponents, while NPCs don't roll and just do damage occasionally.
 
More common examples of this include:
*social skills; systems may have NPCs susceptible to intimidate/diplomacy, whereas PCs might not be – since this infringes on a player’s control of the character and their main input into the game (“deprotagonization”).
 
*attributes: HarnMaster has an intelligence score for NPCs; for PCs this is however just called Memory; while it adjusts skill bases identically for both, there is no pressure on the player to roleplay a character as dumb after rolling a low score.
Slave NPCs in Tunnels & Trolls have no 'Luck' or 'Charisma' ratings (a CHA can be bought if you want an especially pretty slave).
 
*damage: Cadillacs & Dinosaurs tracks hit points for NPCs or monsters as a single total, even though PCs instead have HPs for different body locations (e.g. arms, legs, torso, head). This is done for ease of tracking; it tends to reduce NPCs ability to take multiple shots (for PCs, these stack up more slowly unless they happen to be in the same spot). It may also make them less likely to be taken out through shots to low HP locations (the head), though the system also gives a kill roll vs. NPCs shot in the head (recommended to only cause unconsciousness to PCs).
Savage Worlds PCs also have more wounds than normal NPCs, to make PCs last longer rather than for simplification.
 
*dice rolling/core mechanic: Savage Worlds gives most PCs a 'wild die' whereas most NPCs don't get one - peculiarly making PCs more likely to botch (a double-1), although some skills also specifically fail just on a 1 on the 'skill die' i.e. Repair, Persuasion.  Apocalypse World as noted doesn't roll dice for NPCs (or have attributes); Unisystem in some cases doesn't roll dice for NPCs, giving them an average roll for 'muscle', smarts' and 'combat' (cf. 'Cutting Down Excess Rolling' post).

*character details in general. A few games suggest that characters not be constructed using the PC guidelines- the GM just gives them attributes and skills as they see fit (Savage Worlds for instance). In something like GURPS this is also pretty reasonable since NPCs could theoretically be built with whatever # points anyway, and because the GM isn’t going to be able to exactly build every every NPC).
Some adventures go the extra step of avoiding details of characters deliberately, or modifying them on the go. The classic Harlequin adventure for Shadowrun for example, never gives any statistics for the main BBEG because “give something stats and the players are going to kill it”. The GM is expected to modify statistics and fudge dice rolls as required to make sure the PCs fail when opposing him, until they reach the finale.
Some games may just not include NPC rules. e.g. World of Synnibarr has chargen rules primarily aimed at creating adventurers, and its largely unclear what stats a non-adventuring NPC is supposed to have. Many of its shopkeepers etc. that do have stats seem to be just statted as adventurers, and often 50th level to avoid PCs screwing with them, but judging from its fiction probably some NPCs were meant to be non-super and need saving.
 
*reduced randomization (i.e. of stats): where a PC requires a lot of dice-rolling to generate, GM 'modelling' of a character (short cutting the dice rolls) is likely to construct something with more precision and control than what a player gets. The extremest case here is Mutant Epoch, where race (sometimes), attributes and skills are randomized; if the players want to go see a fixit guy to help repair the ancient vehicle they found, the NPC in question can't be randomly built in the same way as the PCs because they'd end up meeting some crazed gladiator or slave courtesan instead.
Even in a random-roll AD&D game, GMs are likely to often assign individual stats to NPCs (e.g. 'the bandit chieftain has Str 16') instead of rolling up everything. (cf. Character Modelling in attribute generation, pg1).
Conversely though, point-buy systems can give an NPC that's only 'partly generated' an advantage - if there's suddenly a question of whether an NPC can prevail due to minor details (if only they had minor skill X, a particular advantage, etc. - like Combat Reflexes to break out of a grapple, or Quick Draw to make a full attack after being surprised, or an extra +2 to a particular save) then having the GM simply choose this may appear unfair. It may be useful for a GM to random-roll a trait then, even when its ordinarily bought.

*Critical hits; NPC antagonists appear for only a combat then die, so from a fun perspective its great for the PCs to gorily decapitate them. The same is perhaps not as fun for players when on the receiving end, however, and is very annoying particularly where character creation is lengthy or long term play is desired. The simplest handwave for this is that only PCs (or special PCs) get to do crits, as seen in the old optional Critical Deck for 3.5 but this is not completely satisfying from a realism perspective. Known other workarounds for this include:
-Warhammer 1E/2E: here all characters have a number of ‘wounds’; only when these are depleted do characters begin to roll on the critical tables. Characters can be permanently maimed, but there is at least a buffer to prevent mighty heroes being shivved by lucky goblins Rolemaster-style. PCs can also spend a ‘Fate Point’ to cancel a death.
-4E D&D has a surprisingly subtle solution to this. A normal critical hit only does max damage (i.e. 8+bonuses for your longsword, instead of d8+bonuses), unless a character has a magical weapon – which ordinarily gives +d6 damage per plus. While PCs are expected to own a magic implement/weapon virtually without fail, this is very uncommon for monsters, even the human-shaped ones (these get a + to hit that replaces the item bonus as a special monster power); consequently their criticals are lame.
- related to that idea, some sort of confirmation roll can be added based off an attribute/skill which NPCs will not necessarily max out, though PCs probably will.
-FantasyCraft requires characters to spend ‘action dice’ to confirm criticals, making crits (I assume) less accessible to mooks. In a similar vein, TORG gives drama deck cards providing special extra results only to PCs.
-Savage Worlds gives PCs the ability to spend a luck point (‘benny’) to ‘soak’, reducing incoming damage (something PCs don’t usually get to roll to avoid).

*Luck point economies.
-Marvel Super Heroes gives villains 'Karma' like PCs, but they regenerate this in different ways or lose it for different reasons to PCs. This is not entirely non-PC like, existing mostly because of the assumption PCs are a particular thing in the world (heroes) that other characters aren't. The karma system is designed to provide roleplaying rewards even though the character is an NPC - the GM gets Karma for a villain when they gloat over a hero for instance[/FONT]
Other games can deny NPCs luck points entirely or give them less, or have a single 'bank' of points for the GM.

*advancement: NPCs may be assumed to be earning XP the way PCs do (e.g. killing monsters), or may have alternative ways to let them earn XP (many Dragon classes in the AD&D period had weird classes which earnt XP for odd non-combat tasks e.g. gravedigging), or this may just be glossed over. Omege here mentions the idea of
setting NPC's levels off their rolled stats (attributed to Dragon #68 but that actually just gives NPCs a level limit based off stats).

*equipment: 3.x D&D PCs had an "assumed wealth by level" which was calculated using monster wealth and expected encounters to level up; comparatively NPCs just have cash as single monsters (x3 on the assumption some is in consumables) so orders of magnitude less. This contributed to significant disparities in effectiveness between PCs and NPCs at high levels, especially "epic level"(21+).

*magic: magic may be a largely NPC thing in some games (e.g. Call of Cthulhu).

*monster abilities. If PCs can play monsters, they may not get the whole range of potentially game-breaking monster abilities. 3.x generally gave the whole suite of monster abilities. 2E AD&D attempted to start monsters as 1st-level characters, so Large monsters traded in all their monster Hit Dice for +1 hit point per hit die (e.g. a firbolg gets +13 hit points at 1st level) although they also take Large-size weapon damage. 'Adventuring' PC monsters would however get a class, which NPC monsters didn't.

Miscellaneous: NPC personality randomization
*games sometimes have systems to randomize NPC descriptors which essentially come down to RP or player choice in the case of the player characters - i.e. personality traits or other minor traits. D&D sometimes uses random rolls to determine alignment (or in 2E, d100 rolls for NPC 'quirks' like 'spits occasionally' or 'mutters a lot', while Cadillacs & Dinosaurs ( Twilight 2000 ?) interprets playing card results as NPC motivations: Clubs = aggression, Diamonds = greed, hearts = friendly, spades = ambitious, with number determining degree and face cards giving somewhat special results (i.e. Queen of Clubs being 'stubborn' rather than aggressive, Queen of Diamonds being lustful rather than avaricious, etc.) and Aces reversing the result (i.e. Ace of diamonds being generous). In most cases these could be used for PCs, but are unnecessary if the player has their own ideas.
For sets of NPCs, an interesting NPC motivation generator for FATE is the 'Mood of the Room' roll using 4DF:
this reads the plus/minus/neutrals on the dice as different factions to give results like +++-; 'on the surface very receptive, but an agenda or traitor behind the scenes'; or ++0- 'your point of view is most strongly represented but loud minorities exist, some preaching opposition and some isolationism'.

Other Unusual NPC Generation Systems of Note: Post-apocalyptic Adventure "Bring me the head of Frank Sinatra" has an (unofficial) Gamma World expansion of Gygax's Random Harlot Subtable, which allows for features including detailed sentient plant bits, multi-gendered harlots and various robot types.

NPC organizations
See note on the 'push pyramid' for managing organizations here

Quote from: Bloody Stupid Johnson;496055D100 systems.
A d100 system is finely detailed, meaning there are no break points/dead spots for attributes – every point is important, limiting min/maxing. It is slightly slower for multiple rolls (as dice must be paired up as 10s/1s).
D100s handle fine detail very well (exact percentage odds) and is the most "transparent" mechanic to players.
Potentially, a d100 additive system can be sped up slightly by rolling d10+modifiers, with a fail-by-1 triggering a reroll against the ones place.
 
d100 systems occasionally generate additional information using the '1s place' of the d100.
-HarnMaster treats rolls ending in 5 or 0 as critical successes or critical failures, to assess these quickly (if non-intuitively)
-Amazing Engine treats the 1s place as the quality of the result
-Warhammer 1E/2E inverts the attack roll (i.e. 39 would become a 93) and uses this as the hit location roll.

Thought I'd add a few comments.
Top Secret/SI(Designed by Douglas Niles and published by TSR way back in 87, it used all kinds of cool/innovative bits in its design--ads/disads, stun/wound dmg marked as /X, luck points, etc)  pulled two result from attacks for close combat/melee. You'd roll under your Attribute (plus small skill bonus), if you succeed, your damage was based on the tens die (a Price is right/blackjack mechanic) and the ones die determined hit location.

Other percentage systems use doubles on the percentile roll as Special result. Seen some that use exactly = to your Success chance as critical success. A price is right/blackjack variation.

Quote from: Bloody Stupid Johnson;496054Some systems independently vary both target number and # dice (oWoD – particularly Werewolf- and Shadowrun 1e/2e). This unpredictably/erratically increases the number of dice needed to get a given # successes and is an approach that should probably be used rarely. Shadowrun uses d6s and allows target numbers (TNs) over 6 (6s add and roll over).
 
Dice roll systems integrate particularly well, with even damage etc. following the same universal system. Attack roll successes may add to Damage successes giving DEX a percentage flow-through into damage (usually less than Strength since successes are normally converted into extra dice, and have to be rolled again), without the math being excessively clunky.
 
.

There is quite a bit of elaboration or compound systems in this category. For instance, in Silhouette You add your Attribute rating/mod (0 based) to your dice result for your dice pool (based on skill level( (which is made as count highest with bonus for each extra six, plus variants) compared against a variable target number. Last Unicorn games Icon system did the opposite, your Attribute gave you the number of dice in your dice pool (d6), where you count highest, while your skill gave a flat add (it also had a drama die taht could add 6, or give fumble chance). This falls into the old modeling skill/training versus talent/Genius.

Quote from: Bloody Stupid Johnson;496058Here I include systems such as West End Games Star Wars and (for combat) Tunnels and Trolls. A pool of dice are rolled and totalled. A problem with this system is that a character's result varies enormously depending on # of dice in a characters pool – making some tasks very easy or impossible for some characters. To match this, Star Wars uses a safety value in that characters can spend Character Points/Force Points to add extra dice when needed. Star Wars also adds a "wild die"; on this dice, 6s are added and re-rolled.
Later editions of T&T include a success counting mechanism as part of combat as well (where 6s count as automatic 'spite damage').

There quite a few variations on additive dice pools as well. Roll x, keep/count only so many dice are common. EABA uses a count highest three, with some special mods for larger than life heroes.   L5R has its roll x keep y rules, with exploding dice.

A variation of this kind of additive system is adding bonus or penalty dice to your add dice. So if you have 3 regular dice +1 bonus die, you keep the 3 highest, while if you have a penalty die, you'd roll and keep the 3 lowest, etc.

Thanks RobMaudib - I haven't read Top Secret so...what's the Stun/Wound marked /x ? And by using 10s place that would be worked out directly i.e. roll 21 = 2 damage, roll 41 = 4 damage?...so that higher skill directly adds to damage?
 
On the doubles on d% - I'd forgotten that one but I think Rolemaster does that for combat rolls (I think Arms Law uses it for weapon breakage).

Quote from: Bloody Stupid Johnson;496054Die pool systems give characters a varying number of dice to be rolled against
a target number, with "successes" or "hits" counted. The first of these that I know of was Shadowrun.
Die pool systems are particularly good for determining how well a character succeeds, but less good at determining if a character succeeds – probabilities are less transparent and in practice designers tend to allocate # successes required for tasks in a fairly ad hoc fashion. 'botch' chances using 1s tend to do odd things.
Some systems independently vary both target number and # dice (oWoD – particularly Werewolf- and Shadowrun 1e/2e). This unpredictably/erratically increases the number of dice needed to get a given # successes and is an approach that should probably be used rarely. Shadowrun uses d6s and allows target numbers (TNs) over 6 (6s add and roll over).
 
Dice roll systems integrate particularly well, with even damage etc. following the same universal system. Attack roll successes may add to Damage successes giving DEX a percentage flow-through into damage (usually less than Strength since successes are normally converted into extra dice, and have to be rolled again), without the math being excessively clunky.
 
A sub-variant of this is the "match counting" system as used in the One Roll Engine (REIGN, Godlike, etc). To succeed in ORE a character needs 2 or more dice to face up the same number. This provides two readouts a "width" and a "height"; The system is particularly good for doing Hit Location (i.e. a character a rolls 3,3,4,4,4,7 might opt to target a character in their "4" location for 3 successes of damage, or in their "3" location for 2 successes of damage. The system handles differing task difficulty only with some trouble, cannot have "fumbles" (since 1s are "left foot" shots); and at least in Godlike, the additional data available is in practice rarely used for most non-combat rolls. In combat it has the slight problem that base damage and initiative are both based off roll "width" (i.e. shots that will kill someone automatically go first).

One of the very earliest and simplest 'dice pool' systems is Prince Valiant, by Greg STafford/Chaosium. http://www.rpg.net/reviews/archive/9/9189.phtml
You added your attribute to your skill to get your 'dice pool' Which is the number of coins you would flip, heads are successes (Gareth Micheal Skarka used this in his Underworld game as well) My hate of D02 kno no limit!

(and yeah I am system archeology geek:) )

Quote from: Bloody Stupid Johnson;512218Thanks RobMaudib - I haven't read Top Secret so...what's the Stun/Wound marked /x ? And by using 10s place that would be worked out directly i.e. roll 21 = 2 damage, roll 41 = 4 damage?...so that higher skill directly adds to damage?
 
On the doubles on d% - I'd forgotten that one but I think Rolemaster does that for combat rolls (I think Arms Law uses it for weapon breakage).

NP. It used a damage silhouette wit hit boxes, where non-lethal damage was marked as slashes in your dmg boxes (total based on Con), and lethal damage was marked as X's With non-lethal quickly recovered, lethal much more slowly, non-lethal could knock you out, lethal kill, they added etc. much like ORE/Godlike, EABA, and others, but way back in '89.

And yep, higher skill, higher damage potential for unarmed/melee weaps. (firearms were random dice )

Yeah, I've seen the doubles for special success in a couple spots.

Thanks again! Mind if I go back and edit in some of your notes (with attribution to you), to keep info as organized as possible for any prospective designers) ?
(if not that's also fine; I'm feeling pretty lazy...).
 
Checking quickly, Shadowrun and Prince Valiant both seem to be '89, though I haven't found more exact publication dates than that with a quick search.

Quote from: Bloody Stupid Johnson;496205Aside from dice, cards are seen in some games e.g.
.

Castle Falkenstein, from R. Talsorian games, directly used cards for action resolution.   http://www.rpg.net/news+reviews/reviews/rev_1610.html

Card values were from 2-10, the different suits mapped to action types, needed to have right suit to get full bonus. Jokers counted as 15. Brief overview in 4th from last paragraph of that review.

Quote from: Bloody Stupid Johnson;512259Thanks again! Mind if I go back and edit in some of your notes (with attribution to you), to keep info as organized as possible for any prospective designers) ?
(if not that's also fine; I'm feeling pretty lazy...).
 
Checking quickly, Shadowrun and Prince Valiant both seem to be '89, though I haven't found more exact publication dates than that with a quick search.

Sure, glad to make it more comprehensive overview. (Why I responded to indvidual sections)

Great work btw. The general rule on mechanics is, its probably been done before, if only on some web published game:)

Oh yeah, more discussion of open-ending mechanics is probably warranted. Main way to get around flat die-distributions, give remote chance of success, etc. Lots of variations common. (ars magica's stress die, roll d10,on a 1, roll again and double result, keep doubling, a 0 on it is a botch) All die open-end/explode, single die can open-end explode (star wars), etc.  DC heroes used an open-ending 2d10 roll compared to your chart, roll again and add on doubles. Why it had a high handling time in addition to the action+result chart (and loss most of the simplicity of the standard d% action chart ala MSH) though it was rock-solid and industrial strength at least:)

(Column shift idea was common for handling bonuses/penalties in such systems, along with color coded results. GW3rd had the most complicated/detailed overloaded success chart, nearly every effect had a sub table of results with lots and lots and lots of conditions ala D20 (sick, nauseus, shaken, unbalanced, whatevs)

Oh, you should probably describe what attributes are used for/represent (inherent ability in an area, usual possessed by all creatures/entities in a game systems. (not always, early D&D mobs, or don't have all of them.), balanced against skills (typically defined as learned abilities, specialized knowledge.)  And their relationship and weighting. from no significant effect, 1/2 ability (stat + skill), to bulk of ability (Warhammer/old TSR systems) (lots of variation in here including no set attributes in OTE, Fudge, variants of Cortex, etc., as well as no particular skills in like Strands of Fate.)

Cheers! and thanks.
 
Will add section on open-ended rolls (although I think this means I have to go re-read MetaScape and lose San) and further commentary on attributes/ weighting. Yeah seem likes there's no idea so weird that someone won't put it into an RPG, just to be original. (apparently 5E D&D is going to let characters with minimum ability scores pass some checks automatically, like Dragon Warriors was doing in, IIRC, '86).

Quote from: Bloody Stupid Johnson;512413Cheers! and thanks.
 
Will add section on open-ended rolls (although I think this means I have to go re-read MetaScape and lose San) and further commentary on attributes/ weighting. Yeah seem likes there's no idea so weird that someone won't put it into an RPG, just to be original. (apparently 5E D&D is going to let characters with minimum ability scores pass some checks automatically, like Dragon Warriors was doing in, IIRC, '86).

Oh was thinking in the about cutting down on rolling Whispering Vault deserves a mention as it had all character/player only rolls. Pretty cool game.

Also, some discussion of problems related to opposed rolls, because of effectively doubling variance. Was reading about people complaining about FATE,think it was Dresden files, where because of the granular trait range versus the effective 8dF variance, you get some huge variation of results in combat.