Meludia - Listening exercises and to sharpen your ear and music theory skills
ChordU - Extracts chords from any song, integrated YouTube.
GNU Solfege - GNU Solfege is free music education software.
Guitaa.com - Turn ANY song into chords, play along with interactive chords and diagram, transpose, loop, tempo control.
Auricula - Practice your sense of hearing and learn to identify intervals for your musical development.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.