Borderlands seems to be the game on sale this weekend, and it really is a fun game!
Steam has Borderlands on for $24.99 this weekend.
http://store.steampowered.com/search/?term=borderlands
Direct 2 Drive also has Borderlands for $24.99, and Cryostasis for $7.50. It seems well discounted, but I actually have not heard anything about that game.
http://www.direct2drive.com/sale/
Microsoft Games for Windows sale this weekend is Red Faction: Guerilla for$9.99 which is 50% off.
http://www.microsoft.com/games/en-US/games/pages/sales.aspx
Happy gaming.