Xen-FTMS 2 is the new version of Xen-Arts' free 4-operator FM synthesis-powered VST plugin synth, and is designed for the production of microtonal and xenharmonic music.
It comes with 115 patches, 55 microtunings and 56 partials files, and improves on its predecessor by offering a redesigned and smaller interface and new modulators (including an ADSR-syle generator and an LFO). There are tweaks throughout the rest of the synth, too.
You can find out more and download Xen-FTMS 2on the Xen-Arts website. It's available for PC in VST format.