Open source Max/MSP alternatives
PD is available from puredata.info - it looks like it is designed and written by the same person who originally designed Max. This looks to be the closest thing to Max, but I have no idea (yet) how much Jitter-like functionality might be there.
Another free package with an audio focus is Supercollider, available at supercollider.sourceforge.net
And of course Ben mentioned Processing, which is found at www.processing.org