Pianu - Learn to play the piano in your browser.
Guitarix - Guitarix is a simple Linux Rock Guitar Amplifier for https://alternativeto.
Flowkey - The easiest way to learn piano with your iPhone or iPad
GNUitar - GNUitar is guitar effects software that allows you to use your PC as guitar processor.
Playground Sessions - Online piano learning game.
Ampire - State Space Modeled guitar amp/cab/stompbox simulator.