I was wondering if there was a freeware modular synth. I discovered this gem and was surprised that it even has a polyphony.
It's very well-advised concept and it has very good balance between simplicity and flexibility. Once you get idea about difference of master modules and voice modules (and common section), you are able to create simple polyphonic patches. And there is a lot of patches to steal ideas from.
Definitely recommended, only thing you can loose is your time playing with it.