Need for Speed World

Need for Speed World is a massively multiplayer action driving game for the PC where you can race with your friends or compete against thousands of other players in a seamless open world.

Quoted from world.needforspeed.com

The game was released last July 20, 2010 and officially launched as a free download from the Need For Speed World website, with players getting access to all races, pursuits and career progressions up to level ten (10).

After level ten (10), gamers will be asked to pay for a starter pack, which allows them to compete up to level 50 and access a range of power-ups and vehicles.

UPDATE!!!
The Need For Speed World is now completely FREE to PLAY! Please check out their site for more details.

Alien Swarm

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.

Quoted from www.alienswarm.com

I played it. I say it's a decent game. I just didn't have much fun with it. Not that it's bad it's just not my cup of tea.

Download from Alienswarm or Download Mirror