Super Mario Galaxy.
Now, if you read it on the American Wii site the headline is "Cooperative Game Play, a Secular Challenge," while on the British site it's "The Old Issue of Two-Player Play." Either way, Miyamoto explains that Galaxy was originally going to have more cooperative elements (like the second person might have controlled jumping, for instance), but a player would only end up getting in the other's way. By having the second player collecting things on screen and being able to point at objects, Miyamoto envisions parents being able to play with their children and the generations assisting each other in the experience. Considering Mario has always been a solo experience (or a player-at-bat concept), it'll be nice to have a friend or significant other be able to do something other than watch. It's a subtle co-op design choice, but one we can see other games picking up if it works.