PC:
Deus Ex and Deus Ex Invisible War now on steam for 5 dollars (FOR BOTH!) it's this weeks midweek madness deal.
WoW (if you want to get into that sort of thing)
CoD 4 (I ran it on a Pentium 4, 1GB ram and 256 graphics card)
Age of Empires III
Warcraft III and Frozen Throne Expansion
Diablo 1 and II (and Lord of Destruction Expansion for Diablo II)
Titan's Quest (found a 10 dollar copy of it that included the expansion at Future Shop, check steam too)
Torchlight
SW:Battlefront 2
Any and all Half Life Mods (Eternal Silence, Dystopia, Insurgency, Zombie Panic!, etc.)
The Elder Scrolls IV Oblivion (ran this too on my shit Pentium 4 PC)
Starcraft & Brood War Expansion
Freelancer (if you find it you can tell me where I miss that game so much)
Sid Meier's Pirates (I really enjoyed this game... it's just fun.)
Civilization (also from Sid Meier)
Unreal Tournament 2004 (ppl still play the shit out of it)
Doom 3 (I'm sure would run if u can run Source games)
Grand Theft Auto (all the ones before GTA IV)
Left 4 Dead (also ran on my shitty rig. L4D2 probs works for you too and it is Source)
Star Wars Knights of the Old Republic 1 & 2
Jumpgate Evolution (according to their current system requirements this game will run on almst any system. Just need to actually release it now!)
There are probably hundreds more, I just don't feel like listing them all!
Also try looking at websites like http://www.gamigo.com/, http://www.ijji.com/, or any other places that offer free 2 play online games. (I really like ijji they have really well made games)