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