Have you ever been interested in learning more about operating systems, specifically how they work at a very low level? I’ve always been intrigued by hobbyist OSes and have came across a couple of interesting ones that I thought are worth your time to take a look at. Hobby OSes aren’t necessarily the most useful tools from productivity or entertainment standpoint, but rather they serve as useful tools for learning and education. They can also be really fun to play around with and tweak! Here is a list of interesting ones I’ve come across:

• MikeOS - An x86 OS written entirely in assembly that serves as a learning tool to teach how OSes work with well commented code and great documentation.
• xv6 - A simple unix like OS from MIT. This learning OS was developed to help students learn about the low-level details of operating systems and hardware architectures.
• MenuetOS - This one isn't as much of a learning tool as the others, but it is a great "hobbyist" OS. It is written entirely in assembly and features a full GUI and a few simple applications.
• LoseThos - An operating system with a goal of "programming as entertainment". This OS was developed in C/C++ and seems to be oriented toward video games.

There are many many other educational and hobbyist operating systems out there. Check out some of these resources for more operating systems and information on OS development.

• List of Hobby Operating Systems (Wikipedia)
• OSDev.org - One of the best, most comprehensive resources for OS development