A lot of die-hard gamers might have heard of the Alien Swarm shooter game in the past. The game originated as a full-conversion mod for the popular arena shooter Unreal Tournament 2004; completely developed by a score of community developers.

Those same developers were subsequently hired by Valve, the top game publisher that's behind Half Life 2 and Team Fortress 2. While working on projects like Left 4 Dead 2 and Portal 2, in the background they were rebuilding their debut shooter game from scratch.

Alien Swarm

Steam (Valve's gaming network) just recently revealed the existence of that remake, mere days before the game launched last Monday. But the accompanying news was even more shocking. This fully-featured game would be released for free.

What can you expect? Think swarms upon swarms of magnificently brutal aliens and empowering online (co-operative) gameplay. There's not all that much of a story, just fast-packed shoot-em-up levels. This is a game you should play this summer - no matter if you're usually a freeware or retail gamer. The only detectable downside is that this shooter game is only available on Windows PC's.

4 Player Co-Op Killing Spree

Alien Swarm offers two modes of gameplay; the less important offline training mode and the online campaign mode. Up to four players can join up to compete and join hands against the alien horde. In that, it's very similar to Left 4 Dead (2). In other things, not so much. There's bound to be less randomization in the levels, in a way enabling you to learn from failed experiences, and develop tactics to beat the levels.

How difficult those levels will be is entirely up to you. Alien Swarm offers 4 degrees of difficulty; Easy, Normal, Hard and Insane. You'll be assisted in your post-modern quest by over 40 different weapons, with an absurd amount of additional weapon configurations and other unlockable items.

There are in total four unique IAF (interstellar armed forces) player classes to choose from: medic, tech, officer and special weapons. And each of those feature two different characters. Create an IAF squad with your friends and make those aliens regret they got up in the morning.

Free, Open-Source & With Separate SDK

Actually, Steam didn't just announce the game to be free. Yes, you can download the game free of charge. But you can also download the entire source code. This is unprecedented for a major game company like Valve, and rocked the socks off of large parts of the internet. This means that everyone can view the original programming that went into make this game. Everyone can adapt the code, and make spin-offs.

Although the game launched with 7 'official' levels, you can also download a level creation kit. For those interested, this is not the standard kit that you'll see included with other Valve games, but a stand-alone one. This includes a tile-based map generation tool, allowing you to randomly generate rooms in your home-brewed levels.

Are you excited yet? That's right! What do you think of Alien Swarm, or it going open-source from day one? Let us know in the comments!