Deconspectrum is a collection of sound-responsive cubes whose color represents parts of the aural spectrum around them.
Full installation description here.
Individual units consist of the PCB in the /hardware directory (Eagle format) and the ATmega/Arduino software in the /software directory.
The software uses the PlainFFT library by Didier Longueville. I have included the version of the library I used for completeness and accuracy; it is slightly modified to optimize compiled object size.