Left 4 Dead 2
Price: $4.99 (75% Off)
Left 4 Dead taught us that the only thing more powerful than an unstoppable army of the undead is friendship. Both Left 4 Dead and Left 4 Dead 2 are on sale this weekend for $4.99 each or $7.49 as a set. That's low enough for you to just buy it for your cheapskate buddy so he can finally join in rather than trying to convince you to play something else.
If you're completely broke and still haven't picked up the game, fret not; Left 4 Dead 2 is playable for free all this weekend for anyone who wants to try it.