A lot of graphics magic happens in Mac OS X to make the UI go together the way it does. In fact, I’m still scratching my head wondering how its all done so well. To help peek behind the curtain, Apple has provided a tool that at least gives a glimpse into the magic that Quartz performs. It’s called "Quartz Debug" and it can be found in the Developer folder after you install the Developer Tools CD.
When you launch Quartz Debug, a simple window is displayed. Enable the Flash Screen Updates and No delay after flash checkboxes and then use your system as normal. Every screen update is preceeded by a yellow flash which lets you see what Quartz is redrawing. This is most impressive when mousing over the dock with magnification enabled. As the icons are scaled, you can see the exact set of pixels that are getting recalculated.
Of course for most users and developers, seeing this in action is more fun than actually useful. But I found it to be an interesting view into how Mac OS X works. Enjoy the show!