ToAruOS is a hobby operating system project under development by UIUC computer science undergraduates. It is based on POSIX and runs on x86. While the ultimate goal is a microkernel, it is currently a very monolithic kernel. The documentation includes links to guides on OS development.
ToAruOS, a Hobby Operating System
2011-12-01 OS News 2 Comments
I discovered this via
Spoiler if you don’t have telnet: