Recently I've been inspired by several different midi visualization concepts, including the Music Animation Machine Sean Hannifin posted about here, as well as the work Dmitri Tymoczko has done.
I'm working on my own application that takes this on at a slightly different angle. Here's a little proof of concept video I uploaded to youtube:
The audio is 100% Garritan, so I thought it would be ok to post this in these forums.
This video shows one of my works put through a very early alpha build of my MidiSpark application. The Midi file and audio files were both exported as they were from Sonar without any further tweaking. The program attaches and synchronizes the two files. The video is processed from the midi file as the audio plays. Honestly, the uploaded video does not do this justice, compared to seeing it play in real-time.
My plan is for completely customizeable visualizations for each track in the midi file, including size, shape, color, movement and transparency of each represented note, as well as various graphical machinations. All style concepts will also be controllable via note-on velocities and controller information. The sample video has one style across all tracks, but it should be very interesting once I get the multi-style running smoothly.
It's still in alpha testing, but I may be looking for beta testers soon. I'll certainly ask among this community first!
Edit: The second demo has been posted with a number of new options running: http://www.youtube.com/watch?v=ej-CtPVKsH0