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.
miRack - Eurorack-style modular synthesizer for iOS.
Omnisphere - Piano, pad and synth VST for DAW's.
SynthEdit - Visual programming software to build your own synthesizer.