I've been working with the PowerSpectrum code inside of Marsyas, and this led me to read up about Spectral Density on Wikipedia. The PowerSpectrum calculates the Spectral Density of a Spectrum inside Marsyas.
This led me to the very interesting topic of Colours of Noise. I'd heard of white noise and pink noise, but never of Green Noise, or Azure Noise. Wow, my mind has just been blown.