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.
GNU Solfege - GNU Solfege is free music education software.
Metronaut - An app to help you play classical music
LenMus - LenMus is a free program for learning music.
Simply Piano - Fast and fun way to learn piano