Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Musicope - The Piano Game. Contribute to oldrich-s/musicope development by creating an account on GitHub.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Playground Sessions - Online piano learning game.
Omnisphere - Piano, pad and synth VST for DAW's.
Synthesia - A fun way to learn how to play the piano.