Demoing Madronna Lab's Virta which has a unique twist of being a VSTi synth that can be controlled by audio using pitch detection. It also can be used as a traditional midi synth as well as a vocoder and perhaps more.
One thing I've run into is the raw audio seeping through underneath the VSTi, which I don't think is the intended use of the plugin. Not sure how to get around this. I've tried a couple different routing setups, including sending audio from a different track with midi controlling the Virta track. Still the raw audio is audible underneath. This might be how Reaper handles audio going into a VSTi and there's no way around it, or perhaps there's a setting somewhere I am missing.
For those interested:
http://madronalabs.com/products/virta