Quartz Composer is a developer tool that ships with Mac OS X 10.4 ("Tiger") that allows you to access all the nifty graphical powers of Tiger's CoreImage without having to learn how to program. Fooling around with it I managed to make a couple of screen savers, and I can already see that there's a lot more potentially that could be done with it. Quartz Composer has controller patches for midi input, but what I think would be really nifty is if you could use it to create your own iTunes visualizers. I'd love to see something replace the tie-dyed-vomit visualizer that has been shipping (relatively unchanged) with iTunes since it first came out. It was fine in 1995, but it just doesn't show off what the Mac can do now, it's an embarrassment. The current homebrew visualizers aren't much better.
I want someone to make an Atari Video Music Emulator!
Check out some sample Quartz compositions on this Japanese page: (You'll need to have Tiger/QuickTime 7 installed)
Found Via QuartzComps.com