Probably one of the best CO-OP games I've ever played.
On sale now for $28
http://www.newegg.com/Product/Product.aspx...=AFC-C8Junction

8.5 rating from gamespot
http://www.gamespot.com/pc/action/left4dea...ary;read-review

89 from metacritic
http://www.metacritic.com/games/platforms/...left%204%20dead

The single player is fun an very intense but it only last a few hours. This is definately a Multi-player game at heart.