Traveller's Tales' Lego games are platform games that play particularly well in two player co-op. They're fun, casual-friendly, and have great design and successful movie tie-ins. The most recent couple of Lego games have introduced an innovative solution to the problem of two people sharing one screen, a dynamic split-screen mode.
Lego Harry Potter (and Lego Indiana Jones 2 before it) have a dynamic split-screen. If the players are near each other, they're drawn together in one scene. If they wander apart the screen is sliced, two scenes, each centered on a player. The clever part is the cut angle is dynamic and calculated to keep the players relative position to each other apparent.
It's hard to explain, but you can see it in action here where Harry and Ron separate and here where Harry flies up above Hermione. It feels very natural and smooth in the game. If a bit irritating, I felt some compulsion to stay close to my friend to avoid it. Still, a very clever solution to a hard UI problem.