Edited the first post to reflect on what is workable. None of us can really run Killing Floor, and that's lighter on systems compared to the other games.
<_< Well, Killing Floor is based on the UT2k4 engine, so really, if you can run something from that era, just turn the graphics settings down and it should be workable, but OK.
Well, Sven Co-op has been mentioned, which is basically a co-op mod for Half-Life 1. From there, there's Serious Sam, which is a wave-after-wave-of-enemies shooter for one to many. If you can get your hands on it, Rogue Spear (Rainbow Six sequel) is an awesome tactical shooter. Much more realistic in terms of firing/damage than the newer ones, and it's pretty fun. I played it a lot back in the '90's and early 2000's.
Honestly, there aren't many co-op games out there; There's been a pretty big emphasis on competition and the deathmatch/team deathmatch modes reign supreme for the most part until recent history with games like L4D and KF.
Aside from that, you could go emulation (or even hardware if you've still got a working console) if you each own a copy of what you're gonna play. One really good co-op game is Seiken Densetsu 3 (the sequel to Secret of Mana) for the SNES/SFC if you can find it to import. It goes for around $40 or so CAD on eBay, which really isn't so bad - It has a great storyline, and plays a little like Zelda in terms of combat. Of course, the translation patch can't be used with the console, but so long as you own the game in question, it becomes closer to lawful than non in the grey area that is emulation.
