Valve just recently announced Alien Swarm and they also released this game via Steam for free. You really can’t pass up a free game but because it was released shortly after their announcement, many people have been plagued with errors, freezes and other annoying issues. Many are awaiting a patch to allow them to finally play this free game.
Alien Swarm is a multiplayer game that you can play with random people or with just your friends on Steam. You can play offline in offline practice but this game was made for playing with friends. This started out as a mod for Source that works on the Unreal 2004 engine and is a top-down shooter.
Despite its bugs a free game is a free game. The creators could have left this as a mod for Source but instead have decided to release it for free to the public.