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!