Delving into Leopard’s /Developer folder shows that Apple has spent a lot of time writing sample apps for developers to play around with to get to grips with the massive number of additions. In particular, a lot of work has clearly gone into Quartz Composer, whose headline feature is support for custom patches, including those written in GLSL.
I think the last one, called “Security”, was included as a screensaver in some of the earlier development builds. There’s a surprising amount to be found in the developer tools!
On the subject of Quartz Composer, Leopard now includes a large number of “default” compositions in /System/Library/Compositions/. I’m not sure what these are for, but they seem to encompass a lot of image units embodied in QC compositions. The new iTunes visualisers are also in there, along with all 40 Mb of Photo Booth movies. I was a little surprised by that, as obviously it means you can’t free up that 40 Mb of space by just deleting PhotoBooth.
(Incidentally, I notice that Leopard now includes the window’s drop shadow in screenshots taken using Cmd-Shift-4 space. Nifty.)