For non-coder visualists:

"For the uninitiated, Quartz Composer is an ‘environment’ tool which lets you patch together various elements in order to build screensavers, performance environments, and animated windows for use within an application. If you’ve ever looked at an Apple screensaver, played with the new iChat Effects or watched photos float and twirl on an Apple TV, then you’re familiar with what Quartz Composer can do. Its peers are tools like Pd (Pure Data), Processing, and Max/MSP/Jitter, with Quartz Composer heavily geared towards motion animation of processed video and images. Input can be everything ranging from your iSight to XML to audio to a Wiimote, with output being a window on 3d space containing all the sprites, effects and movement you care for. Because Quartz Composer is a real-time system, you can plug stuff together whilst watching and tweaking..."

Hyper Cool + it's free - and they've just released an updated version of it.

(Thanks for the link Greg J Smith :-)