Simply Piano - Fast and fun way to learn piano
Guitarix - Guitarix is a simple Linux Rock Guitar Amplifier for https://alternativeto.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
GNUitar - GNUitar is guitar effects software that allows you to use your PC as guitar processor.
Yousician - Learn to play music at home, at your own pace
Ampire - State Space Modeled guitar amp/cab/stompbox simulator.