Guest post by 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.
I discovered this via
telnet miku.acm.uiuc.edu
Spoiler if you don’t have telnet:
http://miku.acm.uiuc.edu/
Who doesn’t have telnet????