Ask HN: Open Source projects consisting of Beautiful Code to improve hacker skills?

In my ongoing quest to become a better hacker, I'd like to checkout out other's code and not only that but also "beautiful code". With "beautiful" I'm referring to code with great design, interesting concepts, well, really anything that one can learn from. The implementation language doesn't really matter, I guess there a lessons available regardless of the language.

So, which open source code do you think is beautiful?