GUIdebook, a website dedicated to preserving and showcasing Graphical User Interfaces
I came across this website today with an extensive collection of various graphical user interfaces from various Operating Systems dating back to the learly 80's. The site is very nicely organized, making it very easy to find spesific information about whatever system you are looking for. You can find anything from, startup screens to shutdown screens, even sounds can be sampled.
If you are looking for good images to create nostalgic skins/themes, relive the old days or simply curious, this is a really good site.
It also features some articles and links related to GUI design and history.