In 2005 Alex Loscos & Thomas Aussenac, created WahWactor, a VST plugin that lets you control a wah-wah filter through a voice signal.
From the abstract:
Using a wah-wah pedal guitar is something guitar players have to learn. Recently, more intuitive ways to control such effect have been proposed. In this direction, the Wahwactor system controls a wah-wah transformation in real-time using the guitar player’s voice, more precisely, using the performer [wa-wa] utterances. To come up with this system, different vocal features derived from spectral analysis have been studied as candidates for being used as control parameters.
Check this poster for a nice overlook of how WahWactor works.
Visit the WahWactor page for more information and a link to download Wahwactor VST plug-in v0.2.