I'm planning to make my version of "color organ". http://neuroelec.com/2011/03/fft-library-for-arduino/
I tried couple of FFT code for Arduino and found ELM Chan's works best for me. It seems accurate and faster than others.
So I just more less package the library. All credit should go where it belongs.
Original Fixed point FFT library is from ELM Chan, http://elm-chan.org/works/akilcd/report_e.html
A way to port it to the Arduino library and most demo codes are from AMurchick http://arduino.cc/forum/index.php/topic,37751.0.html
Processing app is based on codes from boolscott http://boolscott.wordpress.com/2010/02/04/arduino-processing-analogue-bar-graph-2/
FFT Library and Processing code download : http://code.google.com/p/neuroelec/downloads/list
Code : http://code.google.com/p/neuroelec/source/browse/#svn%2Ftrunk%2Fffft
Here is a demonstration video comparing FFT from Arduino and Ocilloscope
If you have any suggestion, let me know.