I've created a way of controlling the pitch of nearly any digital/electronic musical instrument via audio, sensor data or even just control voltage using a few simple components. The important one is called the LTC1799-it's a resistance controlled squarewave generator that is often used as a replacement for quartz clock crystals.
Fun thing about it is that it is simple to setup and is stable enough that you can use it to slow down and pitch audio devices. Just a few connections and you'll be using your cat's purring to control the sample rate of your synth.
The current implementation is: Analog in--->AVR--->SPI---->Digipot---->LTC1799---->device.
Here are some examples:
I've also figured out that with very few changes one can make a passable ADC/DAC combo that works really well for bit crushed audio sounds.
The code is quite simple too: It's basically (in rough pseudocode)
audio = analogRead(5) >>2 (//for 8 bit pots-10 bit pots this isn't needed, though I guess you could do this by reading adch)
slave high
transfer command bit
transfer (audio)
slave low
I can upload it if people want.