Valve hired on the team responsible for Alien Swarm two years ago. Since then, they've been busy working on the Left 4 Dead series and now Portal 2, but they've managed to squeeze out a little extra time to update Alien Swarm as a full Source release.
Alien Swarm is a top=-down four-player shooter, similar to arcade games of old. Players one of eight unique characters with four different classes, and battle their way through hordes of aliens using more than 40 different types of weapons and equi9pment. Players will gain levels as they play, unlocking persistent items along the way.
Not only is Valve releasing the game for free on Monday, it's releasing the game's entire code base and modding tools as well. With a simple, tile-based map editor, modders should have a field day with Alien Swarm.
Look for it on Steam come Monday!