Operating systems run your computer!
There are lots of innovative OS projects but only a few are really used.
Aside from Linux, Windows, MacOS and derivatives… https://www.ipredia.org/os/
https://mirror.dernuntius.de/projects/wii-linux/ http://songseed.org/dinghy/concept.html reenvisioning the os
Operating Systems: Three Easy Pieces Books
hobby operating system written in C
https://www.reddit.com/r/osdev/comments/husrzq/looking_for_a_specific_tutorial_from_10_years_ago/ macos emulation layer for linux toy os
https://blog.twitter.com/engineering/en_us/topics/open-source/2020/hunting-a-linux-kernel-bug.html demikernel/demikernel: Demikernel OS
nickbjohnson4224/rhombus: a hobby operating system written in C jmgrosen/shorai: A super-amazing OS that will bring world peace. Demikernel
How to Protect Your Privacy on Linux well known operating systems course at upitt just in time specialization for opencl kernels http://c9x.me/articles/gthreads/intro.html trigger kernel races reliably
kabbi/wonderland: Inferno clone to conquer the world
http://intermezzos.github.io/ a demo operatin system for learning https://littleosbook.github.io/ the little book of of development
http://wintermute.org/brendan/ the screenless office and other artistic efforts
https://github.com/mkilgore/protura toy os kernel and utilities https://makelinux.github.io/kernel/map/ map of the linux kernel
https://www.mercuryos.com/ reengineering of the operating system as a ux project, fluid experience driven by human intent
https://github.com/ssrg-vt/hermitux binary compatible unikernel
https://github.com/oasislinux/oasis statically linked linux system https://github.com/dlannan/ljos luajit command line os
https://news.ycombinator.com/item?id=10957020: overview of real alternative operating systems for considering. lots of interesting innovations!