Currently, D&D is very much locked into a certain mindset of "traditional fantasy". As far as actual WotC books are considered, all D&D campaigns take place in a vaguely European medieval society full of knights, elves, orcs, and wizards. Elements inspired by non-European cultures are less common (in fact, anything more specific than vaguely European is less common), guns don't exist (to an extent that counters the historical presence of gunpowder in the medieval world), and the entire world is governed by generic polytheistic gods and magical planes of existence.
Sticking with official TSR and WotC products:
Nonwestern: Oriental Adventures (1st Edition and 3rd Edition versions), Maztica (2nd Edition), Complete Ninja's Handbook (2nd Edition), Old Empires (2nd Edition), Al Qadim (2nd Edition), Empire of the Petal Throne (OD&D related:
Tékumel :: The World of the Petal Throne). There were several Dragon Articles on using India and Africa as campaign settings.
Blackpowder Weapons: 2nd Edition Player's Handbook, Forgotten Realms Campaign setting (both 2nd and 3rd Edition), 3.x Edition DMG, Immortals Storm module (for pre 3e D&D). There are lots of Dragon magazine articles on it.
Modern Weapons and Warfare: The previously mentioned sections in the DMG 1.0 and 3.x. Masque of the Red Death (2nd Edition),
Modern d20 (3rd Edition compatiable), Eberron (using D&D with modern sensibilities), and the Dragon module "City Beyond the Gate"
Futuristic Weapons and Warfare: Buck Rogers XXVc (2nd Edition), Expedition to the Barrier Peaks (1st Edition), Gamma World (all but 3rd Edition and the Alternity version were D&D compatible), Alternity (with a little work compatible with 2nd Edition), and Star Wars d20 (and its subsequent editions Revised and Saga)
And that doesn't even begin to include the 3PP such as Deadlands d20 with its Gunslinger character class or Dragonstar.
I could run an entire Final Fantasy Type Campaign with 2nd Edition D&D and 4th Edition Gamma World, though I would probably throw in a bit of Buck Rogers XXVc just because they all used
the same basic system. And I could do it even easier with the D&D 3.x and Modern d20 games.
And all this stuff spans from the beginning of D&D to this very day.