ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.
Flowkey - The easiest way to learn piano with your iPhone or iPad
Qsynth - Qsynth is a simple Qt4 application wrapper for the ...
Yousician - Learn to play music at home, at your own pace
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.