Ask HN: Open-source projects with inspiring code?

I have seen a lot of open-source projects with different code quality. But sometimes you see code where you think: Wow that is written very well. This can be a few line of codes or the way the whole project is written.

I had these moments when I saw code that is e.g. very idiomatic or very good to follow and understand.

So where do you have similar experience and what do you think caused it?