The terrific STALKER: Shadow of Chernobyl is on sale on Steam this weekend!
75% off!
It's a rough diamond and one of the most atmospheric games ever made. For that price, you have to try it out!
EDIT: I love this game so much that I sent in a reader review to PC Gamer and it got published. That might be the geekiest thing I've ever done, which is saying something.