Editing Druids list:
Multi-player shooter: Battlefield Bad Company 2 and soon Battlefield 3. For a less serious shooter, I would look at CS:S and Team Fortress 2 as well as Monday Night Combat.
Single-player shooters: Crysis 1 / 2, the Half Life series (recommending these again because they're just so good).
COOP Shooter: Killing Floor, Left 4 Dead, Alien Swarm was pretty fun (Free on Steam.. or I think it is)
RPG: TES IV: Oblivion. The best RP I've ever played. Skyrim should be a must have come November. Adding Dragon Age: Origins and Dragon Age II here. Mass Effect and Mass Effect 2 were good, Mass Effect 3 is coming out soon, too. I guess Portal and Portal 2 would fit in this category too.
Zombies: Left 4 Dead 2, Killing Floor, Zombie Panic Source (not a very serious zombie game, but fun and free [as a mod for a source engine game])
Survival Horror: Dead Space 1 / 2, and what carnageX suggested, Amnesia. Also, the Penumbra games are amazing (from the makers of Amnesia)
RTS: Starcraft 2 (can never go wrong here!) If you're looking for different types of RTS games, try out Civilization V and Sins of a Solar Empire Trinity.
MMO: Really isn't much in this section... World of Warcraft and soon The Old Republic.
I don't think it fits in a genre: Garrys Mod. I've been getting back into it recently and I love it
(Just so you know, it requires a Valve game [TF2, CS:S, Orange Box, Half-Life 2, etc])
Also, GTA IV.. It's another game that's just fun to have when you're bored.