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