For fun this last week I made a spectrum analyzer using D3 and the HTML5 Web Audio API. Been meaning to check those out for a while, and spectrum analyzer seemed like the natural way to kill two birds.
Here it is running on Heroku with some of my music as the example track
http://d3-spectrum.herokuapp.com
And the github page here:
https://github.com/arirusso/d3-audio-spectrum
Increasing the curve setting gives the spectrum a more logarithmic display, traditionally more common for audio spectrum analyzers.
The other controls are pretty straightforward
Enjoy
Updates:
Adding live audio input in Chrome Canary (10/4/2012)