i had a play with max msp at uni, very powerful program, and so much you can do with it. A freind of mine created an installation, using light sensors to trigger midi and digital audio, depending on how you moved within a certain space on the floor.

I also used a program on mac called Metasynth, really liked the image synthesis tool, which allows you to take a digital image, and use it to filter or create sound, using the images colour, shapes, brightness etc. as a modulation source. very cool stuff.