Half-Life 1(seriously, no mention of this yet? You guys should be fucking ashamed.)
Half-Life 2
Portal(You should be able to run most of the source engine games actually, theyre retardedly flexible in graphics)
SOF 1 & 2
Stalker: Shadow of Chernobyl (be sure to patch it to the very latest, or else its barely playable.)
SWAT 3 & 4
DooM 1 & 2
Medal of Honour: Allied Assault
Call of Duty 1, 2 and 3
Unreal 2
Here's a few to start out with, you may not be able to run ALL of these, but some will work, HL1 is a sure thing, and i'd be surprised if you couldnt get HL2 running smoothly.