from the documentation:

   Pd is a real-time graphical programming environment for audio and
   graphical processing. It resembles the Max/MSP system but is much
   simpler and more portable; also Pd has two features not (yet) showing
   up in Max/MSP: first, via Mark Dank's GEM package, Pd can be used
   for simultaneous computer animation and computer audio. Second, an
   experimental facility is provided for defining and accessing data
   structures.

WWW:  http://www-crca.ucsd.edu/~msp/software.html

Also see the unofficial Web site at http://puredata.org/.