Chord Master - Build your own chord progression
ChordIQ - Learn notes, chords, sight reading, perfect pitch and more.
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
ChordPic - Create beautiful guitar chord diagrams
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.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.