ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
Chord Master - Build your own chord progression
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
ChordPic - Create beautiful guitar chord diagrams
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Chords in Keys - A simple web app that lets you play the chords for major and minor keys, allowing you to quickly try out different musical ideas and come up with interesting chord progressions.