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.

Unofficial web site: http://puredata.org/

WWW: http://msp.ucsd.edu/software.html