Comes out on Monday and is totally free
Pre-purchase Alien Swarm on Steam
From the store page:
- 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
Video
YouTube - Alien Swarm - Gameplay