Synthesia - A fun way to learn how to play the piano.
Midi Studio - Full-featured, modular MIDI control surface for iOS
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Navichord - Intuitive chord sequencer and MIDI controller for iOS
Playground Sessions - Online piano learning game.
IsoPad - IsoPad lets you use your Windows tablet to play electronic MIDI instruments.