Joakim Frostegård has released a free and open source frequency modulation-based synthesizer plugin for Windows, macOS, and Linux.
OctaSine is a 4-OP synth with parameters for volume, panning, modulation index, feedback, frequency modifiers (ratio, free and fine), envelope values (attack, decay, release) and toggling of white noise mode.
Each operator has an ADSR envelope with logarithmic slopes and a white noise mode which makes it easy to create percussive sounds such as hi-hats and cymbals. The synth features flexible modulation option, including 4 LFOs with multiple waveforms, oneshot and loop modes and optional DAW BPM sync.
Released under the AGPLv3 license, OctaSine is free to download with VST binaries available for Windows and macOS. Linux users can build OctaSine from source.
More information: OctaSine