One of the things that first drew me to Max was its ability to connect such a wide variety of software and hardware in meaningful and flexible ways. Anything from video game controllers to muscle tension or brainwaves becomes fair game as a performance interface.
The Resistor JelTone created by NYC Resistor, a NYC based hackers collective, is one of the most humorous and unexpected interfaces I have seen. Using capacitive resistance fields and americas favorite summer salad ingredient, gelatin, they create novel instruments using Jell-o molds. The Arduino reads in the capacitance data and is passed on to Max/MSP to generate the sound.
Check out this JelTone toy piano: