Not the Doppler Effect
This applet was an attempt at illustrating the Doppler Effect experienced when standing next to a sound source that is spinning in a circle. The attempt was abandoned when I realised that the speeds and frequencies involved in the real situation cannot be accurately recreated in the illustration, which thus becomes more confusing than helpful.
The yellow rings represent the wavefronts of the sound emanating from the red sound source, and the blue dot is the observer. The rate at which the observer is blinking represents the frequency of the sound he hears. (It's a he, trust me.) The intensity of the yellow represents the intensity of the sound he hears. This was important, because in this particular demonstration, people often mistake the varying volume of the sound as the Doppler effect, when we want them to hear the changes in pitch, so I wanted to illustrate the difference.
The problem with this illustration is that for it to make sense visually, the speed of sound can only be slightly faster than the speed of the sound source (whereas in reality it is of course much faster). As a result, the sound takes too long to reach the observer, who thus hears the highest pitch not when the source is moving towards him, but when it's closest to him. And that is precisely what would reinforce many people's misconceptions...
So this is a very bad illustration of the Doppler Effect, but a good example of the limitations of animations as teaching tools.