When Steve Jobs left Apple he founded NeXT computers. They made NeXTStep, an especially well thought out and cool looking GUI. They opened up the NeXTStep architecture and renamed it OpenStep. Then they got bought by Apple. Many OpenStep concepts were incorporated in to Mac OS X Server. GNUstep is a free implementation of the OpenStep development kits for X windows. Window Maker is an X windows manager that tries to achieve some of the NeXTStep look and feel. I found this nifty introduction to the NeXTStep UI.