Came across this GUI gallery today. The site has quite a comprehensive collection of screenshots of different Operating Systems. I’ve seen many sites with Windows screenshots, but I haven’t come across many with Chicago (Windows 95 early builds) which is interesting. There are also some screenshots from a pre-1.0 version of Windows.
I also came across ReactOS, a free software clone of Windows. I’ve tried it before quite a while ago and the screenshots look pretty good, although to me ReactOS seems like not much more than a way to get WINE without Linux.