http://store.steampowered.com/app/630/
Theres a new, free game of VALVe called Alien Swarm, which is pretty much of a surprise as it was announced only three days before it was released, it is completely free and pretty good for that kinda game.
It is from a top down perspective, you fight yourself your way through lots of aliens in a science fiction environment, and it requires quite some teamwork on higher difficulties.
Just played it with three friends together on Mumble (voice communication software like TeamSpeak 3), played till last map on hard difficulty, and only stopped because it was getting late. We had a good time there.