[LEARNING] Well-written open source codebases

ITT we post examples of well-written open source codebases to learn or draw inspiration from. Please restrict your suggestions to codebases that you have actually spent some time reading.

I'm fond of reading bits of the earlier, C-based idTech engines, and they're often held up by others in the blogosphere as an example of good C, so they'll be what I'm recommending.

https://github.com/id-Software

This guy has written some pretty detailed code reviews of the engines that serve as excellent high-level introductions before you start peeking in the source, if you'd like to study the techniques used by the engines themselves and not just the structure of their code.

http://fabiensanglard.net/

I'm interested in seeing some recommendations for dynamic languages like Lua. My idea of how to properly structure a program is limited to idiomatic C at the moment, which makes writing Lua programs feel somewhat awkward to me.

Why not learn some ultra-theoretical pure mathematics and then invent a new form of computing where you then invent a new kind of computer and which then you invent a new kind of programming concept and which then you invent a new kind of language.