I'm the developer of Goldspace, a free and open source UNIX-like kernel that I've spent the last 6 months on! By kernel, I mean that it's the core of an OS. It's under the GPL-2.0 license, so anyone can use it, share it, modify it, etc., and you only need to credit me! :) It's capable of preemptive multitasking, which means it can run multiple programs at once, just like Windows/macOS/Linux can!
It's been fun to develop, so check it out if you want!
https://github.com/Goldside543/goldspace