Ask HN: Which open-source projects are worth studying?

Lately, I've been reading through some of Paul Graham's essays, and last night, I read an essay posted in May of 2003 called "Hackers and Painters".

Here is a link: http://www.paulgraham.com/hp.html

The author suggests that hackers (or, by extension, programmers) can enhance their abilities by studying examples of good programming. He draws a parallel between hackers and painters or writers, highlighting how they, too, benefit from the practice of emulating great work. For hackers, open-source projects are a wellspring of learning potential.

This prompts a necessary question: Which open-source projects are worth studying, and why?

The following is the direct quote from Paul Graham's 2003 essay, Hackers and Painters:

"The other way makers learn is from examples. For a painter, a museum is a reference library of techniques. For hundreds of years it has been part of the traditional education of painters to copy the works of the great masters, because copying forces you to look closely at the way a painting is made.

Writers do this too. Benjamin Franklin learned to write by summarizing the points in the essays of Addison and Steele and then trying to reproduce them. Raymond Chandler did the same thing with detective stories.

Hackers, likewise, can learn to program by looking at good programs-- not just at what they do, but the source code too. One of the less publicized benefits of the open-source movement is that it has made it easier to learn to program. When I learned to program, we had to rely mostly on examples in books. The one big chunk of code available then was Unix, but even this was not open source. Most of the people who read the source read it in illicit photocopies of John Lions' book, which though written in 1977 was not allowed to be published until 1996."

Here is a link to his essays if you wish to read them: http://www.paulgraham.com/articles.html