Pianu - Learn to play the piano in your browser.
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.
Joytunes - Learn music through mobile games
Scale Heaven - Browse and play any scale/chord, chord progression.