Ask HN: Best open-source projects to learn from?

What are the best open-source projects / snippets of code that caught your attention? I understand that there are different criteria when someone classify something as best / beautiful. I mean below properties when I mention the word 'best' -

a) Solves a real problem b) Relatively easier to understand c) Relatively easier to modify / extend d) Relatively easier to test e) Has seen an evolutionary curve over time

I make my living by writing code for past few years & feel like reaching out to the best folks in the industry for their insights.