Unfa says (I think same):
"ZynAddSuBFX is the most complex, powerfull and versatile software synthesizer I have ever seen. What's more it is free (libre) software with source code available. It might seem difficult to program at first, but thanks to a big set of patches shipped with it - new users can first explore what the synth can do, and then experiment with the knobs on their own.
Very good, reliable and powerfull softsynth"
This is a great free synth, it looks confusing, so I would suggest getting the new version called Zynfusion, I don't know if its free or not but you can build it from source, at least you can on linux :p.
It’s an old software that never updated, but I still downloaded a vst version of this plugin from the internet. It’s all FREE download, just loaded the vst in my DAW and messed with some sounds. It’s got a TON of presets, however some of them suck...