Seriously. Left 4 Dead. Get it now. GET IT RIGHT NOW.
Do you remember, a long time ago, playing games that were fun even when you were losing?
Imagine playing a game like that with 3 other people, with randomized levels so nobody can memorize the pattern and just blow through them without a struggle.
Grab the demo if you still need to be convinced. It's only got one chapter, but I played it for like 6 hours straight. There were enough changes each play through to make it an incredible experience.
Left 4 Dead -- xbox 360
Left 4 Dead -- PC
Sorry ps3 people, ain't no party for you.
Also, it's like 6 bucks off right now, so I'd pull the trigger and use the left-over cash to buy yourself a happy meal or something.