Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Flowkey - The easiest way to learn piano with your iPhone or iPad
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Yousician - Learn to play music at home, at your own pace
Omnisphere - Piano, pad and synth VST for DAW's.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.