As far as interesting musical interfaces go, the axis 64 is the most amazing thing I've ever seen from a conceptual standpoint. It's like someone just laid down the same of all of these relationships that make music work and put into something your hands can push on. What's weird is that even though it's just an /interface/ it makes the /concepts/ easy to grasp, manipulate, etc.

http://www.youtube.com/watch?v=D7OeRkXWTtQ