FFTExplorer
It performs spectral analysis on real-time data created by its internal synthesizer.
DOWNLOAD FreeFFTExplorer is an advanced, Java-based signal processing workshop meant to introduce a number of advanced and interesting electronic processing methods.
For those unfamiliar with Java applets, the frame below is not a picture, it it a working copy of FFTexplorer, ready for your experimentation.
To turn off the sound (and see FFTExplorer's true update speed), click the checkbox marked "Sound" at the lower right.