Gamersgate is having a summer sale. This week is the RPG sale. All the games on sale are at least 50% off, some even going as high as 75% off. Not much in the way of co-op (in fact I don't think there are any that fall under the traditional definition of co-op) but many good deals to be had none the less. And if you're interested in Torchlight (the sequel was recently announced to have co-op) it's only $5.