Installed:
Blockland
Crysis
Fallout 3
Oblivion + Shivering Isles
Morrowind + Tribunal & Bloodmoon
Grand Theft Auto IV
Grand Theft Auto 2
Return to Blockland 1.45
Sauerbraten (Cube 2)
Unreal Tourmanent 3
Savage
Phantasy Star Online: Blue Burst
S.T.A.L.K.E.R.: Shadow of Chernobyl
Prey
System Shock 2
Half-Life 2
Half-Life 2: Deathmatch
Half-Life 2: Lost Coast
Half-Life 2: Episode One
Half-Life 2: Episode Two
Portal
Left 4 Dead
Team Fortress 2
Counter-Strike: Source
Day of Defeat: Source
Garry's Mod
I own a few games I haven't installed:
Rayman 2
Supreme Commander
Neverwinter Nights 2
Tony Hawk's Underground
Age of Mythology + Titans
The Sims
Zoo Tycoon 2
Halo: Combat Evolved
Tribes: Vengeance
Guild Wars
I can't help but feel that I've missed something, but I'm pretty sure this is it.
EDIT: My suspicions were correct, I had missed out Guild Wars.