Pure Data (Pd) is the a real-time graphical programming environment for audio and graphical processing written by Miller Puckette. It is the software I primarily work with and you can find more about it at www.puredata.info.
Here is my paper from the Pd Convention 2007 in Montreal/CAN about the ArduinoI board and Pd. Pduino and other Arduino Interfaces for Pd
FFT - Some words about Fourier Transformation in connection with Pd
FFT and PD english
Pd patches - Some abstractions and useful patches
A small jewel for hardcore pd freaks: a vintage (Windows) version of Pd! (And this version indeed was very advanced! Before that I remember Pd used to crash if you typed a wrong name into an object box - I have no backup of those...) Pd-0_20.zip