ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
ChordU - Extracts chords from any song, integrated YouTube.
soundrop - Soundrop is an app that allows you to create your own personalized sound track to keep you focused or relax.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
ChordPic - Create beautiful guitar chord diagrams