ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
Flowkey - The easiest way to learn piano with your iPhone or iPad
Scale Heaven - Browse and play any scale/chord, chord progression.
Yousician - Learn to play music at home, at your own pace
7Pad Scales and chords - Music instrument and assistant for playing scales and chords.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.