Alien Swarm is a short and simple third person shooter offered for free via steam.
Very similarly to Left 4 Dead, the game is a 4 player coop. You could play by yourself, but the bots are too good and you'd be missing 90% of the point of the game. There are 4 classes and each class has two characters which offer slightly different stats for that class. There is only 1 campaign with about 7-8 levels and each level take maybe 5 minutes or less.
I beat the game today with four friends on hard. There's more to do: we could go back and try insane, we could get to max level, we could score some achievements, but there really isn't much replay value. Still it was lots of fun playing through together the first time.