These are the FPS games I play frequently these days:
Medal of Honor: Allied Assault - nice graphics/sound, immersive atmosphere, both single and multi modes are well-designed
Tribes 2 - one of the best games around for multiplayer
Deus Ex - the best single-player experience of any game out there in my opinion, multiplayer is fun as well
Thief II: The Metal Age - great strategic gameplay and challenging difficulty
Unreal Tournament - fast-and-furious multiplayer mode keeps making me come back, this is definitely the game I am best at
Descent 3 - as with MOH, solid singleplayer and multiplayer, hard in both

Doom 2 - great classic game with fun singleplayer
The plot in Deus Ex was exellent, I was so hooked up with the game that I just couldn't stop playing untill I had finished it (ok I DID sleep and go to school, but anyway).
I got this game with my computer; when I first tried it out I played it for nine hours straight without any breaks.

I have played through the game 12 times now and still find it fun.

BTW I actually didn't even know that you could save Paul Denton and Jock until the 7th or 8th time I played through the campaign.
