The idea of a multi-user operating system is almost a tautology today but back in the 1980s it wasn't all that common - at least when it came to personal computing. PC-MOS was a multi-user operating system that, like DR-DOS and others, competed with Microsoft's MS-DOS before eventually disappearing at the Redmond juggernaut crushed almost all its competition. Now, Roeland Jansen, Gary Robertson and Rod Roark have put the operating system onto GitHub as an open source project so we can all mess with its source code.
With so many new computing platforms available to us, like Raspberry Pi, Arduino and others, I think the availability of source code has the potential to unleash a new wave of creativity as today's developers look at code that was designed to run on far leaner platforms than today's versions of Windows, macOS and Linux. That older software could run very swiftly if ported to some of those platforms.
The code published on GitHub includes disk images so you can get a system up and running quickly as well as the source code and documentation.
You'll need a complier - Jansen says the repository he has released "does atm NOT include the borland compiler/tools -- not sure if releasing them here is OK". Although that could change of he gets permission to post the complier although he says it shouldn't be too hard to get your hands on them.
One of the fun projects I did for a story a few years ago, at around the time Windows 7 was released was to fish out the media for every version of windows and DOS I had and create VMs for all of them. It's pretty interesting to look back and through the evolution of software to see how far we've come and how some old ideas have come back into vogue.
I still think iOS looks a lot like Windows 3 - lots of icons on a desktop, arranged into name folders.
Now, to remember how to edit autoexec.bat and config.sys files...