Starting 12am EST tonight, or 9pm PST, for one day, Warhammer 40k: Space Marine will be on sale for $12.49, the lowest it's ever been. I know a handful of folks who have been waiting for a deal, including me. It's a third person shooter with melee action. Story, co-op, and class-based multiplayer. Can't go wrong for $12.49.
If you're unfamiliar with Green Man Gaming, it's a site that sells Steam game keys cheap. Legit and been around a while now, with great customer service. GMG is oft linked/referenced on the Steam forums with no mod disapproval.