Hum - Songwriting for iPhone
Flowkey - The easiest way to learn piano with your iPhone or iPad
Tape It - App for songwriting & audio recording.
Yousician - Learn to play music at home, at your own pace
Songcraft.io - A platform for songwriters to write, organize & collaborate.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.