If you are interested in the evolution of the modern GUI (graphical user interface) you should check out the excellent "Road to Leopard" series on AppleInsider.
More than just a look at the new Macintosh OS, the preview breaks down each component of Mac OS X and talks about the history of each, going all the way back to origins of the application.
The chronology is complete with screenshots to compare the old to the new. It is surprising to see how much difference there is between things now and just five years ago.
