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.
yalp.io - Automatic chords extraction from mp3 and YouTube
Auricula - Practice your sense of hearing and learn to identify intervals for your musical development.
Guitaa.com - Turn ANY song into chords, play along with interactive chords and diagram, transpose, loop, tempo control.