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