ChordU - Extracts chords from any song, integrated YouTube.
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Scale Heaven - Browse and play any scale/chord, chord progression.
Riffstation - The web version is Free and you can learn how to play Chords with Youtube Songs.
ChordPic - Create beautiful guitar chord diagrams