Whether it was an historical event, a tv show, a comic, movie, book, etc, or just something that happened in real life. Whether it was a character, an item, a villain, a monster, a place, or a whole campaign.  Whether you just cut/paste it, or modified it, or it inspired you to do something different but slightly related.

Give us the details.

Tomorrowland.  

I have a Villain named the Broken Lord that is a mesh of Ultron and the computer from tomorrow land.  I also stole Athena, but she calls her self Thea so the Olympian doesn't accuse her of hubris.  

Unfortunately they pulled the Broken Lords head off in the first session.  One of them took the body and head back to the workshop so....

Probably doesn't count but I borrowed characters from the D'Artagnan Romances and the siege from Cyrano de Bergerac recently. But then again, it was for Flashing Blades so I'd be surprised if anyone didn't take material from Dumas, at least.

The carpet was replaced in our office this summer.  We have an off and on D&D game going on there.  While the carpet was still being laid, I switched out an owl bear in an adventure with a "carpet monster" using mostly the same stats, but concealed as a big rug on the floor.  The party was suspicious, because the rug seemed to be in better shape than anything else in the room, and there was nothing on it.  When it attacked, I think my description of the red glinting eyes and the claws pushed the rug part out of their minds.  But even after they killed it, and noticed that it oozed out white/yellow blood that smelled like glue, no one caught on.  

A couple of hours after the session, one of the players put it together.  She was new to D&D, and didn't realize that the GM could make things up whole cloth:  "I thought that sounded like a carpet monster, and it fit our office, but didn't think there could possibly be a carpet monster in your books."

I'm currently using the great forest of Davokar from Symbaroum as the Dim Forest in Greyhawk in a AD&D 1e campaign, complete with massive trees and gargantuan ruins of a forgotten civilization.  A couple of the blighted creatures have made an appearance as well.  I would happily try Symbaroum myself, but this group is not the right place for it.

That period when Pope Benedict dropped out of sight for a couple of weeks came up in a one-shot champions game.

Turns out a heretical faction of the catholic church wanted to sacrifice blood of the pope and the blood of an angelic pc to power DaVinci's god-engine that was in a tunnel under the vatican.

The Pope was sacrificed and a look-alike took over the role for a few weeks until he "Died".

Lord Vetinari.  The PCs were talking to the Lord Mayor of the town, and I steepled my fingers and tapped my chin with my index fingers.

The two players who had read Discworld instantly recognized who they were talking to.  It was awesome.

A few months ago I saw a documentary on TV from the BBC called "The Lost Music of Rajasthan." Incorporated large chunks of it into my current Tekumel campaign, especially when running a bunch of village encounters in North-Eastern Tsolyanu. Really brought the game sessions alive.
For those interested in such things I think it can be on You Tube.

There is an abandoned and decrepit motel that I drive past a couple of times a month that used as the setting for an encounter. It just lent itself to putting the bad guys under enough cover to make them tougher, but still isolated.

Dune.
House Kimboat and House Renquist are engaged in a most secret war, to the detriment of the Imperial Empire.

Just one small section of D&D 5e The City of Crows setting I ran my players through a couple of days ago:

Dead Cells (Early Access Video game on Steam), there's a stage called The Ramparts and I used the scenery for a part of city, where a cataclysm cut the city in by a third called The Tear.  It's a series of towers that sprouted from the seemingly bottomless chasm, each tower is connects by bridges or floating sections of walls.  No one knows where they came from.

Here, a Visual Reference:

[video=youtube;0d5xVUqYE_c]https://www.youtube.com/watch?v=0d5xVUqYE_c[/youtube]

I love the background scenery, and I decided that the entire sector is inhabited by Kenku (See Volo's Guide to Monsters.)

The rest of the city is stolen from other sources, like Dark City, other Video Games and at least ONE comic.  But I'll limit to just The Ramparts.

Necessary Evil has been a game of ripoffs. Unfortunately for me, my players didn't have too many original ideas for their supervillains, so they stole liberally from Marvel and DC. Mongo was basically The Hulk (who then got turned into a Vampire Lich), Das Ubersmench was German Nazi Superman, Chimera was Mystique, Micro was Ant Man. And we had one guy as a Power Ranger as The Verde Ranger. My fiancee made the only original character not based on anything in Bubbles, an Atlantean hybrid focusing on psionics. Chimera didn't last long, not enough combat abilities, so was discarded. The Huntress came next, a Predator ripoff. Instead of hunting Aliens she hunted V'sori. The last session for the character was odd. She was always mute but she talked that episode. During a combat to get a ship, she damaged the ship. Once they were inside the ship in space, she pulled the pin on a fusion grenade. They had to force her out into space to save themselves. They brought her body back into the ship after the mission was over. When they returned to base, they had one of the Lemurian doctors perform an autopsy. They found out that it wasn't the real Huntress. She was a V'sori plant. Then the autopsy went south as an alien parasite came out of the creature and killed the doctor before dashing into the base's ducts. Micro shrunk and fought the creature inside the ducts before it could reach an NPC (Siberian) as the next victim of the alien parasite. The "Alien" was a V'sori bioweapon that had been placed inside their agent, probably mind-wiped by mind reading to not even know it was done to her. It was quite a cool ripoff as it had acid damage field, tail attack, corrosive bite and acid burn spit attacks, armored exoskeleton, darkvision and awareness that required activation, leaping, wallwalker, and growth 2 (the reason it grew so fast).  I also gave it Doesn't Breathe, so it could survive in vacuum, which is how it survived space. I'd put the character sheet up, but its way beyond the size limit for attachments.  But it was a fun little response of a rip off to a rip off. The encouraging thing is that as characters died or retired after that point, every new character from that point became more original.  Setsuna with her decay abilities, Ruin with his father inside a sword that allows him to transform into three alternate elemental forms, and Wheeljack whose children were Mongo's last victims. When he tried to stop Mongo, he ripped his legs off, leaving him without working legs. Wheeljack got some cool stuff like a rocket wheelchair and a mech though to make up for this mobility problem.

Quote from: Steven Mitchell;990493The carpet was replaced in our office this summer.  We have an off and on D&D game going on there.  While the carpet was still being laid, I switched out an owl bear in an adventure with a "carpet monster" using mostly the same stats, but concealed as a big rug on the floor.  The party was suspicious, because the rug seemed to be in better shape than anything else in the room, and there was nothing on it.  When it attacked, I think my description of the red glinting eyes and the claws pushed the rug part out of their minds.  But even after they killed it, and noticed that it oozed out white/yellow blood that smelled like glue, no one caught on.  

A couple of hours after the session, one of the players put it together.  She was new to D&D, and didn't realize that the GM could make things up whole cloth:  "I thought that sounded like a carpet monster, and it fit our office, but didn't think there could possibly be a carpet monster in your books."

Hah!

I adapted the old T.V show Emergency to a Doc Wagon campaign for Shadowrun. It was intersting and I have people begging to run it for them since they missed out the first time.

Cadfael: The monastery, the monks, the sheriff, the village -just about everything, only with the names and serial numbers filed off.