ArchiveOS
link
first wayback machine archive: July 29, 2015
Active: Yes! (content is still being updated today)
Archive of downloads and information about all kinds of operating systems with all sorts of uses. Great for finding obscure operating systems. Kind of a niche website because most people would look to archive.org for this sort of thing, but archiveos has a massive catalog of (more organised) content which most people wont know about.
This site is about an object-oriented operating system written in C++ and made by people at the University of Illinois. The website has many dead links on it, but some work. There is a pretty interesting "News & Updates" section that has some reading material attached to it, and some other content related to software engineering and information about the operating system scattered throughout the website. (yes, I did find this on archiveos)