Werkzeug's Jinja2 templating engine written in Python.

I happened to dig into its source code for something and eventually found myself amazed at how the entire base is beautifully laid out. It's no nowhere near being naive. It's almost a simple programming language implemented in python. A compiler of sorts with its own abstract syntax tree and stuff. Yet the code is very readable, straight forward and just taught me how to write good python.

[edit] corrected typo.