Scale Heaven - Browse and play any scale/chord, chord progression.
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
7Pad Scales and chords - Music instrument and assistant for playing scales and chords.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Scales-Chords - Free online music tools featuring a searchable chord database and chord charts.
ChordPic - Create beautiful guitar chord diagrams