If you like Half Life, try Gunman Chronicles, it runs on the same engine & looks pretty good.
Other good ones:
Blade of Darkness (3rd person swordplay medieval type game)
Ino (3rd person shooter with hand to hand combat)
Hostile Waters (Very good RTS, with a bit of a twist)
Alice (mentioned earlier)
Serious Sam (very intense, no excuses FPS)
System Shock2 (Older game, but very good. one of my personal favorites)
Conflict Zone (I don't know if its out yet, but looks like a good RTS)
EDIT: Just saw a vote for Rune...also an EXCELLENT game. To mchammer187, if you liked Rune, try Blade of Darkness.