Synaesthesia

Check out the wonderful Whitney Music Box, by KrazyDad.

If you liked Electroplankton, you may well find you stare and play with this for a while.

The idea is to turn a harmonic visualizaion (basically dots moving at standard related speeds around a fixed point) into the corresponding harmonic music. The result is hypntoic and beautiful - and very natural sounding.

You can read more about it the theory here.