The game development team calling themselves Black Cat was hired by Valve a while back to work on Portal 2 and Left 4 Dead 2. During this time, they were working on a promising looking title based on Valve's Source Engine entitled Alien Swarm. Once they were actual Valve employees, Alien Swarm was pushed to the side ... until now.
As of today, those with the Steam Client installed may download Alien Swarm for the cost of free. In addition to that, you may download the game's source code and mod it to your liking.
To find out more about the game and how to acquire it, visit AlienSwarm.com.