The displacer beast, a cat-like creature with tentacles coming out of its shoulders, is loosely based on a similar creature called the Coeurl from The Voyage of the Space Beagle. The xill, a race of four-armed marauders that lay their eggs in still-living victims, are similarly inspired by the Ixtl from the same novel.
The derro, a race of degenerate human/dwarf hybrids, were inspired by the Deros, a race of Ultraterrestrials from the writings of Richard Sharpe Shaver.
Many classes can be traced to popular fantasy characters—the Ranger is based on Aragorn, the Thief is the Gray Mouser, the Barbarian is Conan, and so on.
The whole "fire and forget" method of spellcasting is taken from the works of Jack Vance. Other Vancian lifts include the Githyanki race (In Name Only), Ioun Stones, the Robe of Eyes and the 'Prismatic Spray'' spell.
Which showed up in Poul Anderson's Three Hearts and Three Lions (the same source as trolls) before Moorcock's more extensive use.
Though Gary Gygax denied heavy influence from The Lord of the Rings, the original D&D game borrowed a lot of elements all the same: hobbits (later re-named halflings), wights, orcs and goblins, mithril (later re-named mithral), balrogs (re-named Type VI demons), ents (re-named treants), and so on.
The iconic Mind Flayer was inspired by the cover of Brian Lumley's novel, The Burrowers Beneath.
Greyhawk Adventures. The Sea Zombie was based on the zombie/ghosts in John Carpenter's film The Fog. They traveled inland under the cover of fog, and the picture has a Sea Zombie breaking through a door with a boathook, a technique used by the ghost/zombies in the movie.
GAZ3 The Principalities of Glantri: "Freeze! Glantri Vice!" (Miami Vice), diamond loyalty forehead implant Dune's Suk Doctor diamond forehead tattoo).
H2 Mines of Bloodstone: Ruggedo the Gnome King (from the Oz series character).
H3 Bloodstone Wars: Adair and Arthur (Norm and Cliff from Cheers)
I3 Pharaoh: "My name is Maniozimus" ("My name is Ozymandias", from the Percy Bysshe Shelley poem "Ozymandias")
I9 Day of Al'Akbar: baking brownies Kieb-Lar and Kieb-Lor (Keebler elves)
I12 Egg of the Phoenix: Mikael Gorchaboff (Mikhail Gorbachev), Fflanidor Fflem (Fflewddur Fflam in the Chronicles of Prydain) and Mersyn Olan (actor Merlin Olsen)
I13 Adventure Pack I, adventure "The Weird Woods of Baron Orchid". A sign says "I'd turn back if I were you", a reference to the sign in the woods near the castle of the Wicked Witch of the West in the 1939 film The Wizard of Oz.
IM3 The Best of Intentions: The Warren (Paranoia's Alpha Complex) and Yertle the turtle king (from Dr. Seuss)
WG7 Castle Greyhawk: The Plane of Silly and Unused Monsters (Island of Misfit Toys in Rudolph the Red-Nosed Reindeer), bats being used to send messages (Mad Monster Party), the three cooks Larifyne, Mowlenhowad and Kurlenshembes (The Three Stooges: Larry Fine, Moe Howard, Jerome "Curly" Howard and Shemp Howard), Poppinfarsh the Dough Golem (Poppin'Fresh, the Pillsbury Doughboy).
Module X2 Castle Amber. The weird and powerful Amber family members who lived inside the title castle were engaged in constant infighting. They and the castle were inspired by the Amberites and Castle Amber in Roger Zelazny's The Chronicles of Amber series. In the module, one character uses magical Tarot cards. In Zelazny's Amber setting, Amberites use the Trump deck for fortune telling as if it were a Tarot deck. The module also introduces Grab Grass, which can grab hold of creatures. In Zelazny's work, the Courts of Chaos have grass that does the same thing. Also, two encounters in the castle were references to Edgar Allan Poe's short story "The Fall of the House of Usher" and the Norwegian fairy tale "The Three Billy Goats Gruff".
More explicitly, they are based on the Averoigne stories of Clark Ashton Smith, and many of the characters and situations in the module originate in those stories.
2nd Edition had a few as well.
Al Qadim setting boxed set A Dozen and One Adventures (1993) had two of them.
The beautiful pahari (mermaid) Salana had red hair and was forbidden to have any contact with surface dwellers because she was too young. She rebelled against this order and tried to learn about their ways, including investigating shipwrecks. She didn't bother to wear clothes when in human form as she was oblivious to the demands of modesty. Disney's The Little Mermaid (1989), anyone?
Adventure "Eleven baneful Gates". In the city of Al-Anwahr there's an inscription on the remains of a statue: "I am Azaltin! See my works, ye mighty, and despair!". It's from the Percy Bysshe Shelley poem "Ozymandias".
Ravenloft campaign expansion Masque of the Red Death takes it title from the iconic Edgar Allan Poe short story. In the adventure "Red Tide" the PCs can find a statuette of a creature with wings, an obese body and a score of tentacles hanging from its bulbous head. This is a reference to how Cthulhu was portrayed in the Call of Cthulhu game.
The Dragon magazine article "The Royal Heralds", describing a spy organisation that could fit into any campaign, gave the commander of the group the name Sir Kell Fleming. "Kell" as in Sir Vernon Kell, the first head of MI-5, and Fleming as in Ian Fleming.
Dungeonscape included a bouncy ball as part of the gear you can buy. Given one of the authors is Rich Burlew, it is clearly a shout out to Xykon's combination of such an item with a Symbol of Insanity.
The Folchurian Lyrist from Complete Adventurer is essentially an updated version of the first edition bard.
The Cavalier and Thief-Acrobat are based on the AD&D v1.5 classes of the same name.
The 3.5 Source BookTome of Magic introduces the Binder class. The class gets its powers by binding various vestiges with seals. While the source of inspiration is obvious many of the vestiges are taken directly from the Ars Goetia, including their appearance, names and seals.
Word of God: Interestingly, Nimbrese have no word for "Wood" or "Forest," because, despite some open meadows and cleared farmlands, they regard all of Nimbral as a place of trees. So, in essence, "The Word for World Is Forest" (or vice versa), right?