New 4 player co-op shooter game just released by Valve on (Ian's favourite) Steam for free. It's a bit like Left for Dead but played from a top down perspective and with aliens instead of the partially dead. Get it
here.
It's actually really good for a freebie and I get the impression that it's very moddable and open to community creations. Probably a good thing seeing as there is only one campaign at the moment. If you're looking for a new game to keep you entertained for a bit I really recommend it.