PianoMania
The most fun iPad app I've played around on lately is "PianoMania". It's a step-by-step program on how to play the piano. What's cool about it is that when you prop your iPad up on your piano keyboard the app wirelessly reacts to your keyboard playing, streaming the songs for you to play and evaluating you on notes correct, timing, etc.. If the song is hard you can set it to stream just right or left hand notes for practice and you can set it to slow down or speed up as you get better. Each song also has a backing track, which makes adding your part more fun. It also has a lot of contemporary songs and seems to be adding more all the time. So you may end up practising songs by One Direction or The Fray or Psy, but that's OK :). It's free for the first few levels and then you have to pay by the month, six-month or year after that. Effective, though, and plenty addictive.