GUIdebook
   Dedicated to preserving and showcasing Graphical User Interfaces. Includes various images, icons, posters, news items and other related materials.
   http://www.guidebookgallery.com/

History of the AmigaOS GUI
   Dedicated to the preservation and restoration of the Amiga Workbench's history.
   http://www.gregdonner.org/workbench/

In the Beginning was the Command Line
   A history of operating systems, by Neal Stephenson.
   http://www.cryptonomicon.com/beginning.html

OldOs.org
   Resource for utilities, history, and much needed to get many old computers up and running, physically or in emulation.
   http://www.oldos.org/

Operating Systems History
   A Brief History of Computer Operating Systems from the mid 1950s on.
   http://www.cs.gordon.edu/courses/cs322/lectures/history.html

Operating Systems Through Time (8-bits)
   A brief history of 8-bit operating systems.
   http://www.armory.com/~spectre/tech.html