Even though you aren't a big fan of FPSes, The Orange Box is still an amazing deal. 5 games for 30 bucks.

Portal isn't really a shooter though. It's a puzzle game in first-person.

RPG:

Planescape Torment
Fallout 1, 2, 3
Baldurs Gate 1, 2
Diablo 1, 2
Gothic 1, 2, 3
Risen
Dragon Age: Origins
Daggerfall [now a free download!]
Morrowind
Oblivion
Neverwinter Nights 1, 2
Icewind Dale 1, 2



Adventure:

Myst 1, 2, 3, 4, 5
Dark Fall 1, 2
Penumbra 1, 2, 3, 4



FPS/TPS:
Call of Duty 4
Left 4 Dead
Battlefield 2
Dead Space



Space Combat:

X2
X3
X3: Terran Conflict
Freespace 2
Freelancer
Eve Online [MMORPG]
Vega Strike



RTS:
Civilization 4 /w Expansions
Heroes of Might and Magic 5 /w Expansions
League of Legends (or is this considered something else?)
Demigod
Supreme Commander /w Expansion
Starcraft /w Expansion
Warhammer 40k: Dawn of War /w Expansions




MMORPG:
World of Warcraft
Age of Conan
Warhammer Online
Lineage 2
The Old Republic
Star Trek Online
Stargate Worlds