The Pong Clock
I rescued an old Thinkpad ages ago and began a project to install it in a picture frame.
The original idea was to have it running things like Conway's Life and the Mandelbrot set. But then I saw the Pong Clock on a blog and decided to just do that instead.
You can grab just the Pong Clock screen-saver (.scr) file and/or the source code (Visual C++ 6.0, MFC as a dll) here.