Meludia - Listening exercises and to sharpen your ear and music theory skills
muted.io - A magical collection of interactive music theory tools & visual reference pages for musicians and music producers
GNU Solfege - GNU Solfege is free music education software.
Chordify - Chordify turns any music or song (YouTube, Deezer, SoundCloud, MP3) into chords.
Auricula - Practice your sense of hearing and learn to identify intervals for your musical development.
Scale Heaven - Browse and play any scale/chord, chord progression.