Darwin is the core of MacOS that is open sourced. It's not really usable out of the box but this project attempts to change that.

This project is really cool, if you want to try it out you will have to email them to get some credentials (which is awkward because human interaction) but it's a unique project that started off based off of amiga. It's completely cloud based and self hostable.

ReactOS is a project that aims to create an open source OS that maintains binary compatibility with windows allowing you to use windows software and drivers flawlessly in a secure and open source system.

After 2 decades of work it is working on windows server 2003 compatibility, but development is speeding up, and it can run programs generally pretty well up to XP, and with little current support it can sometimes run win7-10 programs like spotify. It's one of my favorite projects to follow and I hope others find it as cool.