Paul Bishop has used the simple Arduino tvout library, a microphone and simple external circuitry and some clever Arduino code to make a real time spectrum analyser that displays its results on a television monitor. The purpose of this is to graphically display the magnitude of an input signal versus frequency within the full frequency range that can be measured. The results are shown in the following video:
Although you won't get the same range as commercial test equipment, building this version is simple and still could be quite useful. Instead of fiddling with an electret microphone, consider using our MIC sound input module:
An interesting alternative display method would be with our super-bright LED Dot Matrix Displays, which would look great at a party or background visuals in a musical environment:
But in the meanwhile, to get started check Paul's blog here.