Well, there's plenty coming and one really close.
You got "Left 4 Dead" which is four-players on purpose coming real soon. And also RE5 is a two person co-op for the entirety of the whole game online.
One of my favorite co-op games is "Tales of Symphonia"(GC/Wii) or "Tales of the Abyss"(PS2) if you are into the RPG thing. It's a really cool action RPG, the whole thing can be played with friends all controlling different characters within the combat sections (lots of them) and there's lots of hidden goodies for you all to find. Real fun game, one of the few JRPGs I like. One guy has to be the designated driver of sorts, just walking around and getting you to plot points and combat segments. But when combat happens, you all control your own character and its real cool.