Ask HN: Most useful GitHub repos to learn from or to use as reference?

I'm always looking to expand my knowledge of CS fundamentals, JavaScript and best software engineering practices.

I have several Github repos bookmarked that I find super useful and have learned a ton from...

online CS courses: https://github.com/prakhar1989/awesome-courses

frontend related case studies: https://github.com/andrew--r/frontend-case-studies

cloning Medium.com with with different frontend + backends: https://github.com/gothinkster/realworld

literally a CS university online: https://github.com/jwasham/coding-interview-university

How about you?