FilterOrgan is a VST/AU effect plugin which adds bandpass filters to your
audio track real-time, based on the MIDI notes it receives.
The filters' frequency is tuned to the received MIDI notes' frequency
so you can play chords and melodies with the filters.
Visit filterorgan.com for more information.
CamTheremin is a freeware program simulating the instrument "theremin" with a webcamera: Touch the sensor lines (drawn into your webcam's picture) to control pitch and volume of the generated sound.