Pure Data - Pd (aka Pure Data) is a real-time graphical programming environment for audio, video, and graphical...
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
VCV Rack - A cross-platform modular synthesizer.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
SuperCollider - A real time audio synthesis engine, and an object-oriented programming language specialised for...
Omnisphere - Piano, pad and synth VST for DAW's.