VCV Rack - A cross-platform modular synthesizer.
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
miRack - Eurorack-style modular synthesizer for iOS.
Korg Legacy Collection - Korg Legacy Collection is a suite of VST instruments that acts as an audio plugin software for integrating synthesizers and effects units into your digital audio workstation.
SynthEdit - Visual programming software to build your own synthesizer.
Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.