Here's another lesser known homebrew OS, Snowdrop OS.
So far, it looks very promising, tinkering with it seems fun.
Has BASIC interpreter, but programs can also be linked/compiled.
Source code is available, too.

I'll let the author speak for himself here:
"Snowdrop OS was born of my childhood curiosity around what happens when a PC is turned on,
the mysteries of bootable disks, and the hidden aspects of operating systems.
It is a 16-bit real mode operating system for the IBM PC architecture.
I designed and developed this homebrew OS from scratch, using only x86 assembly language."

Link: Homepage

PS: The author's main site is neat, too! I higly recommend browsing by.
Lots of electronics projects, some of which interface with Snowdrop OS.
Examples:
- stepper motor driver controlled by Snowdrop OS
- interfacing a Nintendo NES from Snowdrop OS
- interfacing with a 16x2 LCD via parallel port

Another one, Solar OS .
It's an experimental 64-Bit real-time OS (formerly 32-Bit).
The OS is written in ASM and has a lot of development tools, it seems.

Requirements:

Only 8Mbytes RAM (even less)
P1 class CPU at 266 or even lower
Can be setup to run with under 2Mbyte RAM with some limitations.

Homepage: http://www.oby.ro/os/

Hi again, please excuse for the double post (tripple post?) here, but there's another retro OS.
It's SerenityOS, a mix of Win95 GUI and Unix architecture.
It's especially made for friends of both Windows 9x/NT and Linux, I suppose.

https://serenityos.org/

There's a thread at Vogons, which was made by me (?) - I'm a bit surprised and confused, to be honest. 😅
Re: New Linux distribution that looks exactly like Win95

Jo22 wrote on 2025-06-14, 00:20:

Hi again, please excuse for the double post (tripple post?) here, but there's another retro OS. It's SerenityOS, a mix of Win95 […]
Show full quote

Hi again, please excuse for the double post (tripple post?) here, but there's another retro OS.
It's SerenityOS, a mix of Win95 GUI and Unix architecture.
It's especially made for friends of both Windows 9x/NT and Linux, I suppose.

https://serenityos.org/

Just as I had my hopes high for an eventual Windows-looking environment to run retro games from the 90s era (if this OS would also supply tight integration of DOSEMU and Wine to run DOS/Win apps like on real Windows), I read their FAQ, and it felt largely discouraging:

Will SerenityOS support $THING? Maybe. Maybe not. There is no plan. […]
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Will SerenityOS support $THING?
Maybe. Maybe not. There is no plan.

When will you implement $THING?
Maybe someday. Maybe never. If you want to see something happen, you can do it yourself!

Where are the ISO images?
There are no ISO images. This project does not cater to non-technical users.

So it feels like someone is just having fun with this and might drop development forever at any time when they lose interest or it works "well enough" for them on their own machine.

Except if maybe someone else picks this up with a more serious and goal-oriented approach.