Midi Studio - Full-featured, modular MIDI control surface for iOS
Simply Piano - Fast and fun way to learn piano
Navichord - Intuitive chord sequencer and MIDI controller for iOS
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
IsoPad - IsoPad lets you use your Windows tablet to play electronic MIDI instruments.
Yousician - Learn to play music at home, at your own pace