Toccata and Fugue http://www.youtube.com/watch?v=ipzR9bhei_o
"Little" Fugue http://www.youtube.com/watch?v=pVadl4ocX0M
Also enjoyable: Debussy's Clair de lune http://www.youtube.com/watch?v=LlvUepMa31o
Back in college (1996 or so -- it was on one of the first PowerPC macs) I put together an animation for a class project that was sort of a combination of the two using Bach's Invention #13.
The view was of traveling down a tunnel of a sort of swirly green fog. Each note was represented by a little yellow or magenta stripe painted on the inside of the tunnel, based on which "part" (i.e. hand) was playing it. The angular position was determined by the pitch (which wrapped around at each octave), and the length of the stripe was determined by the duration of the note. All in all it was a really effective way of visualizing the song.
I wish I would've videotaped it, because the source code is long gone and wouldn't run on modern hardware anyway.
Emulation could have worked.
http://en.wikipedia.org/wiki/Graphical_scores
*Here you can read more about Solitude, that crazy looking graphic at the top of the wiki article: http://at.or.at/hans/solitude/