So, let's say we want to min-max a summoner.

Of course the first step is SF: Conjuration and Augment Summoning.

What else can a play do, with feats or otherwise, to improve its summons?

Is there any feat that allows people to apply the Celestial template to all summons?

It depends on the DM, but since Fiendish and Celestial templates have the same effect on a creature's CR, I would regard the creatures in the PHB tables as illustrations more than as an exhaustive list, and allow a caster free mix and match between template and base animal. Just ask yours what his take is on that point... Then again, there's the fact that the spell level does not necessariuly match the monster's CR, and the fact that some monsters that would seem eligible, or even desirable (like the succubus) don't appear anywhere... so still tread lightly

Thaumaturgist PrC from 3.5 Dungeon Masters Guide.

There's a feat in Mongooses Encyclopaedia Arcane - Conjuration that will allow a arcane caster to meet the requirements for thsi class.  Plus it has many other goodies for maxing summoning.

Additionally the Alienist PrC from Complete Arcane is also a good one to boost summoning and power of things summoned.

I had a Sorcerer 8/Alienist 10/Thaumaturgist 5 that completely kicked ass.  

Miniatures Handbook has a bonded summoner that gives you a constant summoned companion.

For feats, two of the best are Mastery of Madness and Mastery of Twilight Denizens (E-PGtE).  The first lets you add the pseudonatural template to all your summons without having to dabble in Alienist.  The second lets you automatically end any summon that brings a creature from Lamannia.

If you have a way to add it to add Summon Nature's Ally to your spell list, the best summoning feat by far is Greenbound summoning (FR-LEoF), which adds the Greenbound template to all nature's allies you summon.

Rashemi Elemental Summoning (FR-UE) is another good one, it adds two special templates, one to air and one to earth elementals.  With those, summoning elementals becomes much more worthwhile.

Extend is usually a must, and Rapid Spell and Mobile Spellcasting get around the inherent difficulties in being a summon and having most of your spells be full round actions.

Arcane Thesis to Summon Monster or Summon Nature's Ally is a must once you have a good Summon you'll use consistently.  Free Rapid, Extend, or even Enlarged spells add to your overall versatility.

As a Wizard a must is Signature Spell for a Summon, or Spontaneous Summoner as a divine varient because you typically need a lot of monsters on the field.

Twin is never a bad spell, nor is delay, but both are pretty costly for your average summoner, as throwing down your biggest or near biggest spell is going to be better than two or three of the lower ones most times.

That's off the top of my head, there are probably a few more.  Your strongest summoner is typically going to be a druid or cleric with animal domains and a neutral alignment.

Janos, Just where do you take Mastery of Madness and Twilight Denizens from? Your acronym pointed me to Player's Guide to Eberron, but that is obviously wrong, as those feats don't appear in that tome's index.

I'm not sure I agree about Extend, as the higher spell slot makes it a trade of getting a weaker creature in exchange for having it stay longer, and my experience tells me that if you really need summoned help, you'll get more use from powerful creatures than from weaker ones, unless it's for destroying really low end foes. And what in heck do you expect a twinned summon to do for you? unless you're facing a spell caster who might try to dispel the summon or otherwise banish your creatures, and you really NEED your summoned beasties to stay, the +4 slot just makes it too costly to be interesting, given the ratio of what you'll be able to summon over what you are going to meet.

Arcane Thesis, Rapid Spell and Mobile Spellcasting, Free Rapid, Extend, or even Enlarged... wow... That's another big lot that I can't remember at all where they are from (and I've checked my usual volumes... PH, complete series, a couple FR ones... but none of those in my library. In the same vein, could you please remind me where the bard beguiler is from?.

Quote from: SvartalfJanos, Just where do you take Mastery of Madness and Twilight Denizens from? Your acronym pointed me to Player's Guide to Eberron, but that is obviously wrong, as those feats don't appear in that tome's index.

Player's Guide to Eberron p. 125-126 under Manifest Spellshapers.

QuoteArcane Thesis, Rapid Spell and Mobile Spellcasting, Free Rapid, Extend, or even Enlarged... wow... That's another big lot that I can't remember at all where they are from (and I've checked my usual volumes... PH, complete series, a couple FR ones... but none of those in my library. In the same vein, could you please remind me where the bard beguiler is from?.

I'd suggest another look in the Compelte books.  Rapid Spell is in Complete Divine p. 84, Mobile Spellcasting is p. 111 of Complete Adventurer.

Beguiler and Arcane Thesis are both found in Player's Handbook 2.  Arcane Thesis allows you to apply Enlarge, Extend, or Rapid spell without the level adjustment to a specific spell and also gives a nice +2 caster level.

In regard to Extended or Twinned monsters, it depends heavily on what you're using the monster for.  Several monsters provide special abilities that are valuable over time, or valuable in the number of applications you can get out of them.  At times you may need monsters for things other than smashing potential as well.  In regard to your question about Twined specifically for example, fighting a horde of smaller creatures, the more you have on your side, the more hits they can soak, additional targets they provide, number of times they can try to hit an incorporeal creature, number of targets they can set on fire, number of fires they can put out, etc.

Numbers themselves can be a significant advantage in the right circumstance.

alright... rapid+ mobile, that's wicked.

As for PHB2, I see... that one is not on my buying list for a long time... I think my budget will be tied with some Eberron and FR books (notably Xen'Drik, Moonshae and Lost Empires), plus likely the MMIV and FCII when it comes out... that should keep me in play until march 2007 or thereabouts...

and I should have suspected you had special applications in mind rather than the basic "combat muscle" one.