EarMaster - Professional software for ear training and practical music theory on Windows and Mac.
Violin by Trala - Violin by Trala is a music learning app that is designed specifically for Violin users.
Meludia - Listening exercises and to sharpen your ear and music theory skills
Metronaut - An app to help you play classical music
GNU Solfege - GNU Solfege is free music education software.
Flat - Online collaborative music score and tab editor, accessible from any device