Winster is a very good cooperative game site. There are about a dozen types of games available and all encourage cooperation. There are several different levels of membership including a FREE one. You will also accumulate points which you can use to receive online gift certificates for things such as pizza, music downloads from Amazon, fancy chocolate, expensive steaks, etc. I've been using them for several years and I highly recommend them.
BTW, my name on there is Skycladlilith. Feel free to add me as a friend