The Plan9 Source code

https://github.com/0intro/plan9

A Professor of Computer Science gave a paper on how he uses Linux to teach his undergraduates about operating systems. Someone in the audience asked 'why use Linux rather than Plan 9?' and the professor answered: Plan 9 looks like it was written by experts; Linux looks like something my students could aspire to write.

But see also the HN thread :

Code which every programmer must read before dying

https://news.ycombinator.com/item?id=2466129