Valve has a present for you, one that's alien themed and can be played with friends.
What do Team Fortress, Counter-Strike and Day of Defeat have in common? The answer is that they all started out as mods that were later made into full games by Valve. Now you can add Alien Swarm to that list, with the announcement that Valve is releasing a new version of the game for free on Steam tomorrow. The game has four-player co-op, a leveling system, and more than 40 weapons. Valve is also updating the Source SDK, so that if you want to try putting levels together for the game, you have the tools available.
Alien Swarm began its life as a total conversion mod for Unreal Tournament 2004, created by Black Cat Games back in 2008. Valve hired Black Cat and set it to work on Left 4 Dead series and Portal 2. But somehow Black Cat found a way to fit in work on a new version of Alien Swarm and as you can see, it looks pretty impressive.
Source: Rock Paper Shotgun