Today Steam is releasing Alien Swarm, which will cost you nothing.
Two years ago, Valve hired a group of modders behind the popular Unreal Tournament 2004 co-op mod called Alien Swarm as ggmania reports. The team has since contributed to popular games such as the Left 4 Dead Series and Portal 2, but on the side, they were hard at work on a Source version of Alien Swarm, and the fruits of their labor will be released on Monday