Sjoerd van Kreel has recently released a free semi-modular synthesizer instrument and effect plugin for Windows.
Previously named SevenSynth, the InfernalSynth features a per-voice (up to 32) oscillator with classic, noise, DSF, Karplus-Strong and mixed-classic generator types, phase, frequency, ring and amplitude modulation, hard sync and unison support. It comes with a selection of per-voice and global effects (filter, delay, waveshaper, reverb), LFOs, CV routing modules, envelopes, and more.
In instrument mode, InfernalSynth is a polyphonic synthesizer where the combined output of all voices is routed through a global effect bank.
In effect mode, the voice generators are not used, and instead an external audio source is routed through the same global effect bank.
The open source InfernalSynth is currently available as a VST3 plugin for Windows only.
More information: Sjoerd van Kreel