For some AMAZING free games, check out Dystopia, Alien Swarm, and Pirates, Vikings, and Knights II. The first and third require that you have a source game, but it sounds like you do (the Orange Box).
If your computer can handle it, Shattered Horizon is an incredibly neat game that's pretty cheap.
As for other paid games, Steam has a large portion of all of the PC games made in the last few years, so you may as well just be asking for any sort of PC game recommendation. That's really just far too big a pool to give a good recommendation, especially without any realy knowledge of your tastes.
Edit: Saw your post about KOTOR. Do it. In a second. The sequel is also good, though probably the buggiest game I've ever played (tip: don't overwrite saves, just keep all of them in case something breaks, you need to go back a bit, and you don't feel like restarting the entire game). Dragon Age is basically fantasy KOTOR, so if you like KOTOR and haven't played it yet, I'd really recommend taking a look at that.
And OH YES, Steam has PSYCHONAUTS. You owe it to yourself to play that game if you haven't.