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Xv6 is a pedagogical implementation of UNIX v6 that MIT created for an operating systems course.

https://pdos.csail.mit.edu/6.828/2012/xv6.html

Minix (mini Unix) is a teaching operating system that goes with the well known os book "Operating Systems Design and Implementation". We used it at Cal. Very fun.

was about to post a link. Minix is great.

http://www.buildyourownlisp.com/contents

This comes to mind:

https://cheatdeath.github.io/research-bittorrent-doc/