[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.

>>9,16
Most game engines are horrible messes of software. In comparison, the id Tech engines are very well designed and consistent. Since most engines are obviously going to be closed source forever, the id Tech engines are possibly the best we have to study.

Perfect example, the guy who did the id Tech code reviews just did one for the Build Engine (Duke Nukem 3D). It did amazing things for its time, but god damn what a hideously designed program it is.

http://fabiensanglard.net/duke3d/index.php
https://github.com/fabiensanglard/chocolate_duke3D