"PD" (http://pd.iem.at/) stands for Pure Data and is a graphical Computer Music System written by Miller S. Puckette .
It works in a similar way as Max (A Computer Music and MIDI system for the Macintosh from Opcode) or jMax from IRCAM.
PD is open source software, it is free for any use and can be downloaded from the internet. PD runs on Win32, IRIX and LINUX platforms.
It is easy to extend PD with third-party plug-ins (so-called "externals"). Many collections of externals (so-called "libraries") are already aviable.