Let's see-
Thief-Nothing like sneaking and sulking, plus listening in on conversations about the Bear Pits, etc. "They don't make bears like they used to..."
System Shock 2-Alas I've only played the demo, I plan to get the full version soon. Very freaky, better then the original.
Deus Ex-One of the best done games I've ever played, perfect blend of all of the elements and has a great feel. Plus being able to take multiple approaches to a problem.
Red Faction-The singleplayer game didn't live up to half of what it could have been, however it's engine is very powerful yet definately underused by Volition. I like this because of the potential it has, if used right.
Wolfenstein-Zombie bashing might not be my thing, but levels like the V2 rocket one hit a soft spot. The right blend of Indiana Jones, and the original Wolfenstein. (I'm talking about the "Nazis, I hate those guys" Dr. Jones, not the cheesy Temple of Doom one)
Tribes 2-Best multiplayer _ever_
STV: Elite Force-Fun, mindless shootfest. I need to get my hands on the expansion set for the Captain Proton(tm) missions

Max Payne-I have to agree with the fact that it's a little too "moody" and full of attitude, bullet time is sweet but sometimes the game is a little too "involved" for my taste

HalfLife-I've been spoiled from playing FPS games like the original System Shock and honestly don't agree with how "revolutionary" it was. Elements that led to HL were there all along, it was only a matter of time

Now the mods done for HL *are* impressive like CounterStrike.