Resistance Fall of Man (the first one) is a decent co op campaign, forget Resistance 2 unless you have more than 2 people, the co op is no good.
One particular jem I love to play with my friend is Unreal Tournament 3. We always have a riot playing CTF where we are on opposite teams with the max number of AI for backup, which is in itself hilarious since they are all absolutely USELESS - example: they cant drive for shit so it creates some good comedy situations. If you do get it, download the Titan expansion (it's free), more maps and you can turn yourself into a 30ft juggernaught which goes nuclear when killed. This is the definiton of awesome.
Something a little more out there would be something like Resident Evil 5 co op, that way your partner is controlled by someone with half a brain instead of the computer which has an obsession with the pistol, using it to the exclusion of all else and it is as much zombie killing as you can handle, as I take it you have played Nazi Zombies to death (excuse the pun, and the mentioning of the pun)
I have other sugestions of things coming out soon (Reach, Fear 3 and so on) but thats a discussion for another time - hope this helps!
M-M-Monster Kill!
