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.

link: http://puredata.info

Some tutorials:

http://www.pd-tutorial.com/english/index.html

http://www.ekran.org/pd/

http://www.obiwannabe.co.uk/html/sound-design/sound-design-all.html