Most of the games have been mentioned here already, but I will give a quick run-down of what Ive played/been playing that I think is good.
Everquest-even though the graphics are dated, and some of the elements still suck after two years Im still hopelessly addicted(I played UO for a year before trying EQ).
Unreal Tournament-same as EQ, a little dated, but still one of my all time favorite games.
Deus Ex-Just an all around awesome game in the first-person.
Hitman:Codename47-another awesome first person game that requires more thought, with great graphics and a decent storyline.
Diablo2-I played Diablo for a year, I certainly wasn't going to pass up this title even though it seemed like it was delayed forever.
Giants:Citizen Kabuto-picked this title up based on Gamespot and friend's reviews. One of the most orginal first-person-adventure hybrid games Ive ever seen, with beautiful graphics to match.
I also have Motorcross Madness 2 which I couldn't get into as much as the first one, and Icewind Dale, but no time to play it.
Games I want to play, but havent picked up yet: Halflife:CS, Alice, Sacrfice, Baldur's Gate II.