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...
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.
SuperCollider - A real time audio synthesis engine, and an object-oriented programming language specialised for...
Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.