MikeOS
A very limited and simple operating system written in assembler for 286 computers (and newer compatible architechtures, obviously). Pretty much what I would imagine as a successful outcome if I would write one to see that I can do it. It works, there's a text editor, a game, you can list, edit and execute files. Not much more though at first glance. I didn't look into writing additional software for it myself, yet. There are many forks of MikeOS. Most of them named after the forker and not under active development. It's a project I'd look into if I'd want to learn x86 assembler. Simple, not looking like any other OS I've seen.