RTM/Z80
RTM/Z80 is a multitasking kernel, built for Z80 based computers, written in Z80 assembly language, providing its users with an Application Programming Interface (API) accessible from programs written in the C language and the Z80 assembly language.
It is intended to be a simple and easy to use learning tool, for those who want to understand the tips and tricks of the multitasking software systems.
This is certainly not the only hobby operating system for Z80-based computers, but the more the merrier.