muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Flowkey - The easiest way to learn piano with your iPhone or iPad
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Yousician - Learn to play music at home, at your own pace
Scale Heaven - Browse and play any scale/chord, chord progression.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.