All of the Half Life series. All of the Unreal Tournament series. Call of Duty 4 Bioshock S.T.A.L.K.E.R. ETQW Roller Coaster Tycoon 2 + Expansions That is all I can think of atm.
Unfortunately, I don't have the best graphics card in the world, so my gaming on the PC isn't too extensive. But, there's nothing I'd love to play more than my copy of Final Fantasy VII on the PC! I am a big fan of the PS1 version, and I can do so many edits to the PC version that make it feel like a new experience. Trainers ftw.