http://store.steampowered.com/app/630/

Today, Alien Swarm was released on Steam as a completely free game to play. 4-player co-op that's essentially a mix of Left 4 Dead and Smash TV. Extremely fun when you have a good group, and pretty atmospheric.

Also, it has an achievement that gives you a TF2 Hat, so even if you just like playing TF2, you've got an excuse to give this game a shot.

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