Nathan Lineback’s highly recommended GUI Gallery has some nice shots of more rare and obscure interfaces, along with dozens of great links to other sites.
MUIseum: The Museum of User Interfaces by Natan Cohen seems a little empty at the moment, but already has a nice section on people involved in GUI development, as well as various version of Windows.
Stefan Berka’s www.operating-system.org is a German website with many information and screenshots of various operating systems (including those GUI-driven)