Steam Sale – all of Shadowgrounds for $5
You can get both Shadowgrounds games for $5 total. They are similar to Alien Breed, Alien Swarm, Alien Shooter, and Alien Syndrome.
For $5 both games are pretty fun. They remind me the most of Alien Breed and are better than Alien Shooter, another cheap Alien killing game.
The link to the sale is the following: http://store.steampowered.com/sub/552/