When it comes to famous operating systems for the Z80 and similar Zilog processors, the first and maybe only one to come to mind is CP/M, which was even made its presence known on the dual-CPU (8502 and Z80) Commodore 128. Yet Zilog also developed its own operating system, in the form of the comprehensively titled Z80 Operating System with Relocatable Modules and I/O Management (Z80-RIO for short). With limited documentation having survived, Ralf-Peter Nerlich has set out to retain and recover what information he can on RIO and the associated Programming Language Zilog (PLZ) after working with these systems himself when they were new.
Catchy name, and awesome work to try and recover as much about it as possible.
Here’s another interesting Z-80 OS: https://www.youtube.com/watch?v=5jTcWRN8IbA&ab_channel=Zeal8-bitComputer
from the comments on that HackADay article, someone has made an MCZ-1 emulator https://github.com/ivanizag/izilogmcz
so people without the hardware can try out RIO etc.