The Clavilux 2000, a gizmo invented by Jonas Heuer, turns any piece of music played ona keyboard into a dynamic piece of information art. It's a pretty simple set-up: The midi-signals from the synthesizer are run through vvvv, an free tool for creating live visualizations. The outputs are then visualized with a projector:
What you'll notice in that video is that every note played appears as a stripe of color. Length and vertical position show the force of impact, while width shows how long the note played; the colors correspond to various tonalities. (Discordant notes are marked in contrasting colors.) Thus, the more tonal variations a piece has, the more colors will show up. When a piece is over, the visualization remains, as a visual summary of the music that was just played--what notes were played most, which were loudest, and what harmonies the piece revolved around.