OctaSine: a free and open source frequency modulation synthesizer pluginhttps://www.octasine.com/

• vst

• vst

libre synth vst

Surgehttps://surge-synthesizer.github.io/

• vst

• vst

A good open source syth plugin

Archie3d/aeolus_plugin: Pipe organ synthesizer (VST plugin)https://github.com/Archie3d/aeolus_plugin

• vst

• vst

Pipe organ vst