Spinamp was my hack for the Music Hack Day San Francisco 2012
Spinamp is a full-featured, skinnable WinAmp clone that runs inside Spotify. It's got a working spectrogram and custom visualization, and even the buttons work.
Thanks to Daniel Kennett for providing me with a special build. The visualization code is based on this JS1K demo by Steven Wittens.
Attention: Spinamp requires custom events currently not available in any public version of Spotify, which means it will probably not work for you.
I hope Spotify adds this functionality to the production version soon, and I hope I can extract the visualization code into its own app.