battlefield 2142 was my favorite for YEARS, and still have alot of dedicated servers running. Not sure what this will be like now that Bad Company 2 is out, but its a really fun game that I got into for a long time. It's a FPS but is really cooperative.
Thats funny because I really liked 2142 and I ended up playing it for quite a while. I read a review about BC2 where they trashed 2142 for some reason. I know it is pretty different for the BF series but I didn't know people hated it that much.