With the release of Leopard, there is a completely new screen saver option. It's really hidden. I've been using Leopard since it came out, and I just ran across this today. You get to it in the normal fashion: Go to System Preferences, then select Desktop & Screen Saver and finally select the Screen Saver tab.

Once you are on the screen saver tab, select a folder of pictures under the Pictures select a folder, then (and here's the key), you need to select the far-right option under Display Style (see the image below). If you hold your mouse over this, it'll tell you this is the "Mosaic" option. But aside from that, there's not much to tell you this option even exists.

When you select it, the screen saver will show a picture from your library. It'll then start panning out showing you that the initial image was one picture in what is an emerging mosaic. Once the mosaic generation is done, it morphs into the picture that was used as the source the mosaic. It then starts panning out again and the whole process repeats.

This is actually a really cool little nugget in Leopard, I'm surprised Apple didn't do something to make it a little more obvious that this feature exists.

The button you need to press to get the Mosaic screen saver in Leopard.