Alien Swarm
So I saw this new game this morning (Alien Swarm) and it is free of charge so I just clicked download. Turns out, IT IS FREAKING AWESOME.
Alien Swarm is a 4-Player Co-op game (like l4d) that has levels like Call of Duty 4 and teamwork that you have never seen! There is a canvas map (meaning you can draw on it and draw out plans as a team) and you can weld doors shut! There are also sentry guns, freeze rays, assault rifles, chain guns, shot guns, electricity rifles and many more awesome things…
I recommend you ALL go download it off steam and play it. IT IS ABSOLUTELY AWESOME!
The one down side is that there aren’t enough maps at the moment (theres about 6 all up) but there are modding tools and I am pretty sure Valve will keep updating it.
PS: It gave me an alien hat in TF2 :D