Over the last few months I've been developing an app in my spare time which uses the Echonest music analysis API's to generate levels for a tap style game. The key differentiator is that you can import any piece of music into the app and tap along. I'm pretty happy with the result and the app is now is the Apple store. The apps name is Tune Cruiser and you can find it by hitting the link below. I get the feeling that developing the app has been the easy part, now the long hard road of app promotion begins!
More info on this project is on the landing page over http://www.tunecruiser.com/.