Valve released a free game on Steam called Alien Swarm and if you haven't tried it, I highly suggest you do. Though there is only 1 official campaign more can be downloaded and played (made by others) and it has quite a bit of replayability.
Valve released a free game on Steam called Alien Swarm and if you haven't tried it, I highly suggest you do. Though there is only 1 official campaign more can be downloaded and played (made by others) and it has quite a bit of replayability.