alien swarm - Valve's free game

You must have known Valve, It's a famous company by its first product, Half-Life and  its distribution software, Steam. Several days ago, July 19, Valve released a free game - Alien Swarm.

Alien Swarm is a free top-down shoot-em-up by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004, and it was developed by the original team, which was hired by Valve during the course of the development process.

• Tactical, 4 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
• 4 unique classes and 8 unique characters
• Matchmaking, Steam Cloud, Steam Stats
• 64 achievements
• Tile-based map generation tool
• Powered by Source and Steam
  
  

It's awesome! Just download it and have fun!