Alien Swarm is a top down, 3D, co-operative multi-player shooter in very similar vein as Left 4 Dead. It also looks a whole lot like the new Laura Croft and the Guardian of Light, a game that costs $15. Alien Swarm provides a simplistic interface, but also includes editor and scripting tools allowing 4 players at play at a time. The game really requires a full contingent to enjoy fully, although bots can be enabled through the scripting tools.
Unlockable weapons and upgrades, four different character classes, and cross-game achievements made for great way for Steam to get a few extra installations of their software to the masses. For the price, it is worth your time.
If you haven't, check it out on Steam. http://store.steampowered.com/app/630/