TouchDAW - Full-featured DAW control and general purpose MIDI control app.
Synthesia - A fun way to learn how to play the piano.
XotoPad - MIDI instrument controller with chords, scales, faders, and more for multi-touch Windows devices.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.
Midi Studio - Full-featured, modular MIDI control surface for iOS
Playground Sessions - Online piano learning game.