Guitarix - Guitarix is a simple Linux Rock Guitar Amplifier for https://alternativeto.
Synthesia - A fun way to learn how to play the piano.
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.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.