Krazydad's Escher screensaver.

I found this a few years ago, lost the install file somewhere in the bowels of my computer, found it again just a few days ago.
Krazydad has taken scenes from MC Escher's artworks and used them to create a kaleidoscopic screen saver. Hard to describe just how mesmerising it is, you'll have to try it for yourself.
The link for the Windows install is at the lower right of this page. Higher up he has a link for Mac users.