Sniper Elite: Nazi Zombie Army
If you have £9.99 spare, and you also have three friends who also have £9.99 spare, I highly recommend you all get on Steam and purchase Sniper Elite: Nazi Zombie Army for some serious four player co-op action fun.
WW2? Snipers? Zombies? What more could you ask for!!!
While it seems to be getting bad rep by the reviewers, it's actually great fun playing co-op with mates and is a bit 'Left 4 Dead'. The more of you there are, the more the zombies come. There is an awesome selection of sniper rifles, and it looks nice as well. :)