If you haven't yet, you should check out 'Alien Swarm' which is a free download through Steam. (Free account required) It's quite possible that you can get it other ways as well but I know it's free, and free multiplayer, through Steam.
Anyway... it's a zombie swarm game (similar to crimsonland, i ma3d a gam3 with z0mb13s in it, and many others) but it's 3d. You have the choice of playing from the standard 3rd person view, or for added difficulty you can do first person, no seeing behind you!