Somebody (not me) made this with a nifty little freeware app that generates animated displays from MIDI files. I find it really helps me keep track of the different contrapuntal voices. Pretty fun.

Other visualizations I'd like to see: Ockeghem's Missa Prolationem, Hovhaness's Prelude and Quadruple Fugue, Josquin's L'homme Armé masses, Pärt's Cantus in Memoriam Benjamin Britten, Baude Cordier's Tout Par Compas, and Bach's Goldberg canons.