Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.
Synthesia - A fun way to learn how to play the piano.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Playground Sessions - Online piano learning game.
Synth1 - Synth1 is a software synthesizer. Functionally it is modeled on the Clavia Nord Lead 2 Red Synth.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.