Not to nit-pick, but I'll append these to GreyArea's list based on the books in my collection (and avoiding games that didn't hit mass market in some manner):
High Fantasy -- pretty well covered (smothered?) -- LL, D&D, etc
Sword and Sorcery -- well covered -- LL, DDC, D&D, etc
Post Apocalyptic SF -- well covered -- Mutant Future, Gamma World, The End, WasteWorld, Apocalypse World, Barbarians of the Aftermath, Aftermath!, Twilight:2000, deadEarth, the Morrow Project, Living Steel, After the Bomb / TMNT, GURPS Autoduel, The Price of Freedom, Alpha Omega
Horror -- well covered -- Call of Cthluhu, Cryptworld, Chill, World of Darkness games, Trail of Cthulhu
Time Travel -- well covered -- Time Master, Continuum, Doctor Who (3 different RPGs), Feng Shui, GURPS Time Travel, Time Drifters, Time and Time Again, SpaceMaster: Time Riders, TimeLords, TimeShip, TimeStream, Transdimensional TMNT
Space Opera -- over-covered -- S&S, Star Frontiers, Traveller (and another 100 or so RPGs)
Super Heroes -- Well Covered -- Heroes/Champions, Mutants and Masterminds, etc
Cyberpunk -- covered -- Cyberpunk 2020, Shadowrun, CyberSpace, Cyber Hero, Ex Machina, GURPS Cyberpunk, Cybernet, Digital Burn, Corporation
Godlike -- sparsely covered -- Sandman, In Nomine, Nobilis, Scion, Immortal, Nephilim, Mortal Coil, The Primal Order, New Gods of Mankind, D&D4e, BECMI D&D, Lords of Creation
Humor -- covered -- Toon, Hackmaster, Paranoia, Red Dwarf, Teenagers from Outer Space, Tales from the Floating Vagabond, GURPS Discworld
General -- covered -- GURPS, GORE, Heroes, etc
Wild West -- sparsely covered -- Boot Hill, Deadlands, Aces & Eights, Werewolf: the Wild West, d20 Past, Western Hero, GURPS: Old West, Dust Devils, Gunslinger
30's Adventure -- well covered -- Indiana Jones RPG, GURPS Cliffhangers, Gangbusters + a million "pulp" RPGs
Steampunk -- sparsely covered -- Space 1899, Etherscope, Airship Pirates, Victoriana, Iron Kingdoms, Castle Falkenstein, GURPS Steampunk and Steam Tech
Spy -- covered -- Top Secret, Mercenaries, Spies and Private Eyes, Ninja's & Superspies, Spycraft, Corporation, Conspiracy X, James Bond, Espionage