1. It called TempOS. It is a kernel for educational purpose too.

http://tempos-project.org The project is listed in OSDev Wiki too http://wiki.osdev.org/Projects 2. Here's another nice one: https://fosdem.org/2014/schedule/event/04_uk_escape 3. take a look at that [0], it's a small unix clone designed for learning purpose. It also comes with a pdf guide which documents the implementation. [0] http://pdos.csail.mit.edu/6.828/2012/xv6.html 4. http://www.oxkernel.com