EarMaster - Professional software for ear training and practical music theory on Windows and Mac.
Playlist Machinery - Tools that help you create & organize your Spotify playlists
GNU Solfege - GNU Solfege is free music education software.
Heartbeat - Listen to music with your friends together, on Spotify
LenMus - LenMus is a free program for learning music.
Magic Playlist - Get the playlist of your dreams based on a song