Fuzix is a very simple UNIX clone (and a for of UZI) started by Alan Cox, an ex-Linux kernel developer) as his retirement project.
It aims to run on old CPUs like Z80 and on microcontrollers. I found it when searching what OS can I run on the $4 Raspberry Pi Pico.
« Oh, and don't be fooled by the archived status - it moved to
Seriously, it's not that hard for the maintainer to write one sentence describing what the project does.
That doesn't tell me what it is in a concrete sense. Must be hundreds of OS that run on similar hardware - what makes this one different?