I stumbled across this site last night and it's too good not to get its own thread.
http://www.co-optimus.com/It's essentially a searchable database that will tell you if a game has co-op multiplayer or not. It even tells you if it has local or online and stuff. Pretty handy tool that I thought some folk here would be interested in. It has pretensions of being a news/reviews site as well, but just use the search bar at the top right to find a game and then you'll be presented with a handy summary page. For example, Trine 2:
http://www.co-optimus.com/game/1883/xbo ... ine-2.html