Great co-op, Tomb Raider+Diablo+Robotron
Of course, it's not a "true" Tomb Raider game, but I wish there were more games like this! Besides this and the Lego games I mentioned above, I can't think of many great local co-op games, yet I find such games offer the most rewarding experience with my daughter. It's really not fair competing with her (someday she'll beat me, but right now she's 8 and can't), but we really enjoy playing as a team.
Also, I applaud this game for being challenging and satisfying but not too difficult to finish. We've actually finished twice.
It also has a surprising amount of content for a $15 download game. Cut scenes are either comic book drawings or brief 3D animations, but that's to be expected to keep the download size reasonable, and they are fine.