Xolotl is an open source "harmonic" synthesiser plugin available for Mac OS (VST3 & AudioUnit) & Windows.

Harmonic oscillator

The “oscillator" consists of up to 32 independently pitched waveforms. These can be standard synth waveforms, multi-samples or single cycle waveforms. By default the pitch increments through the harmonic series, but can be un-quantised using the step knob..

Flexible 16 Stage Sample and Hold

32 waveforms need at least 32 sample and hold effects, right? maybe not under normal circumstances, but this one's a bit different. Depending on how it’s configured, it can act as an effect, sound generator or modulator. 

32 Stage Multimode Filter

A bunch of filters with independent control of the cutoff. There's a ton of sonic potential here. Comb filtering, phaser effects and some very tasty interaction between the S&H section.

Finally..

At the end of the signal chain is simple ADSR envelope and pan spread control.

Modulation

Xolotl has 2 looping envelopes & 3 over-spec’d LFOs with mixable waveforms and unique waveshaper 

Sound Library 

Over 200 samples from classic like the Oberheim Xpander, Roland System 100, Roland JX-3p and more. 

Source code and build instructions can be found in this GitHub repository.

Please note that Xolotl is under active development, it is not a finished product and there will be bugs :).