Wade is right!
...Miller Puckette--the inventor of Max/MSP--has written a similar environment to Max/MSP called Pure Data which is available through open source. I've tried on several occasions to use PD in lieu of dropping the cash on Max/MSP, but have always come running back to the commercial product. The reason for this is simple: Documentation. Both environments have documentation, but the Max docs are much better and more complete. Furthermore, if you run into a really big problem with Max you can call the company directly and sort it out. This isn't so much an option with PD unless you happen to be on a first name basis with M. Puckette (chances are that you aren't since I hear he's very reclusive). Lastly, PD is audio only and doesn't deal with video (sad face). So if you're tempted to go the free route with PD, consider yourself warned.