muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
ChordU - Extracts chords from any song, integrated YouTube.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Guitaa.com - Turn ANY song into chords, play along with interactive chords and diagram, transpose, loop, tempo control.
Scale Heaven - Browse and play any scale/chord, chord progression.
ChordPic - Create beautiful guitar chord diagrams