Simply Piano - Fast and fun way to learn piano
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
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.