The Sonic Smith Squaver P1 is a semi-modular, audio controlled analogue synthesizer. It uses a proprietary chip designed (patent pending) by Sonic Smith to give you super fast audio control of the analogue oscillator. It also adds PWM, sub oscillators, harmmonics, VCF, expression pedal input and foot-switches to make this a really unique product.
The demo used a microphone to control the synth, but you could just has easily use a guitar or something else entirely.
More info at Sonic Smith
Using Chains and automation to switch keyboard setups