Few good games are on sale right now on Steam;
Left 4 Dead - $4.99
Post Apocalypse game where you and three other survivors have to make your way through different levels to a rescue point.
Killing Floor - $4.99
Fend off wave after wave of different specimen until you fight the Patriarch. Leveling up your character along the way; based on Unreal Tournamanet 2004's Unreal Engine; you can play the UT2k4 Mod version of this game if you have UT2k4 for free; but the stand alone game is so much better. Well worth the $5. Lots of DLC you can buy too, but I'm not interested in the offerings they have (Different playable characters and a weapon pack)
I picked up these games on a Steam sale about a year ago. Well worth the money IMHO.