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