Pd (aka Pure Data) is a real-time graphical programming environment for audio, video, and graphical processing.
Pd is free software and can be downloaded either as an OS-specific package, source package, or directly from CVS. Pd was written to be multi-platform and therefore is quite portable; versions exist for Win32, IRIX, GNU/Linux, BSD, and MacOS X running on anything from a PocketPC to an old Mac to a brand new PC. Pd can run on smartphones thanks to projects like libpd and RjDj. It is possible to write externals and patches that work with Max/MSP and Pd using flext and cyclone.
Pd was created to explore ideas of how to further refine the Max paradigm with the core ideas of allowing data to be treated in a more open-ended way and opening it up to applications outside of audio and MIDI, such as graphics and video.
It is easy to extend Pd by writing patches (abstractions; programs within the Pd-language) or object classes (externals; using one of the many language bindings, like C/C++/lua/python/...). The work of many developers is already available as part of the standard Pd packages and the Pd developer community is increasingly growing. Recent developments include a system of abstractions for building performance environments; a library of objects for physical modeling; and a library of objects for generating and processing video in realtime.
For more information visit the website for the project.