VOSIMSynth is a free, fully modular audio synthesizer with a focus on speed, audio quality, and ease of use. Check out the github page (https://github.com/austensatterlee/VOSIMSynth) for more information.
Pre-built binaries for Windows can be found on the github release page, or directly from KVR. Note that these links are currently 64-bit builds only.
Nightly builds (including 32-bit builds) can also be obtained from AppVeyor by visiting the github page and clicking on the AppVeyor badge at the top of the readme.
Also, please note that VOSIMSynth has mostly been tested using (and therefore likely works best with) Reaper and VSTHost / SAVIHost.