AmpToMIDI turns the amplitude of an audio signal (e.g. your voice, a guitar) into MIDI control data.
This means you could control the volume of a synth using the expressive power of your voice, or increase the amount of reverb on your guitar signal the louder you play. There are many possibilities, hopefully you will find far more than have been detailed here.
AmpToMIDI is a SuperCollider standalone. Which is part of the reason that the interface is a little... ugly. This should improve soon with the release of the QT GUI for SuperCollider
This only runs on Mac OS X. It has been tested on 10.6 and 10.7