This is the Color Organ / Spectrum Analyzer from
http://fftarduino.blogspot.com/2011/02/color-organ-spectrum-analyzer-on.html

It implements fixed point fft on the Arduino.  Currently it outputs to an 
LED strip and lights up a series of red blue and green depending on frequency
range. 

Audio input is expected on A0.  You can use something like http://www.sparkfun.com/products/9964 or http://www.sparkfun.com/products/9868 or just build your own offset circut with a mic or line in.