Alien Swarm is a four-player shoot em up, where you play in a team of marines charges with cleaning out the attacking extra terrestrials. It's a pseudo 3D top-down shooter, which has really good production values for a free game!
Valve Corporation, the people behind Portal and Left 4 Dead, have brought Alien Swarm's blend of classic arcade action and modern graphical touches together for free. You can set up your own games or join public ones easily, then choose your marine class from Officer, Special Weapons, Medic and Tech. You play in teams of four, and it's usually best if you have one of each type.
While Alien Swarm's gameplay is top down, the action is remeniscent of Left 4 Dead. You are similarly faced with an enemy that rushes at you and need to work together to survive difficult odds. The graphics are sound are great for this type of game, with cracking weapon effects and atmospheric, claustrophobic environments.
While it won't change the world, and lacks any interest if you don't want to play online, Alien Swarm is a surprisingly good free gift from Valve. The difficulty levels will challenge everyone - easy is suitable for beginners, but may still kill them off! Games are tight and exciting, with enough variety to keep you coming back.
Alien Swarm is a great free multiplayer shooter, with excellent gameplay, sound and graphics.