MMURTL - Message-based Multitasking Real-time Kernel
This little operating system was written by Richard A. Burgess in the early 90s and written about in the book "Developing your own 32-bit operating system". The code has since been placed in the public domain.
I bought this book in 1996 while I was still in high school and it was one of the most influencial programming books I've read. And still is. I learned a ton from this book and from this code and spent the next three years learning about operating systems and compilers.
For anyone interested, here is the full source code.
The book is available for free in PDF format, along with the code and updates (all of which have been merged into this repo). http://www.ipdatacorp.com/mmurtl/