Valve is using its Steam online distribution hub to provide PC gamers with access to ‘Alien Swarm’, a new squad-based tactical shooter. The whole thing is absolutely free and an official release over at SteamPowered.com explains:

“‘Alien Swarm’ is a game and Source SDK release from a group of talented designers at Valve who were hired from the Mod community. Available free of charge, the game thrusts players into an epic bug hunt featuring a unique blend of co-op play and squad-level tactics. With your friends, form a squad of four distinct IAF Marine classes.”

Along with the game you get the complete code base for ‘Alien Swarm’ that features updates to the Source engine as well as the SDK. The company statement adds that ‘Alien Swarm’ adds “third person camera, depth of field, improved dynamic shadows and a wide variety of game-play additions to the Source engine”. A few features of the game are listed below:

• Tactical, four-player co-op action game with a top-down perspective
• Complete game code and mod tools
• Unlock persistent items by gaining levels
• Over 40 weapons and equipment with countless loadout configurations
• Four unique classes and eight unique characters
• Matchmaking, Steam Cloud, Steam Stats
• 64 achievements
• Tile-based map generation tool
• Powered by Source and Steam

Head here for some more info. Enjoy!