Killing floor is 3.99 and free to play for a week. Pretty much the most replayable game I own. I have like 250+ hours into it and always go back for more. It's absolute, pure gameplay. No story, cutscenes, or other fluff, you just jump in and start killing. The class system is great, with each class filling certain niches, the community is surprisingly awesome, and there's a pretty high skill ceiling in the later modes (suicide, hell on earth) that definitely weed out the pros from the chaff. It's pretty much a 6-player co-op wave based arcade shooter, and each class is good for killing specific types of monsters or doing certain things.