PD
"PD (aka Pure Data) is a real-time graphical programming environment
for audio, video, and graphical processing. It is the third major
branch of the family of patcher programming languages known as Max
(Max/FTS, ISPW Max, Max/MSP, jMax, etc.) originally developed by Miller
Puckette and company at IRCAM. The core of Pd is written and maintained
by Miller Puckette and includes the work of many developers, making the
whole package very much a community effort.
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." so... PD branched off from Max/MSP a few years ago but was started by the same developers and is still opensource and free! Its not as pretty as Max/MSP but it is a lot very flexible. It has lots of examples of Audio Synthesis, Midi interaction and Video synthesis and manipulation. There is also a networked session functionality which means you can connect to other people using PD over the internet and alter each others patches and have live jams.
Basically its like Max/MSP but free, open source and a lot more geeky. The best download I've used is below. It contains all the necessary files plus loads of externals already installed like GEM, the video object. http://at.or.at/hans/pd/installers.html

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