I wrote a parser and lexer by hand. Building the mental abstractions to figure out how a syntax tree should be constructed and walked was amazing. It gave me the tools to visualize how code is structured, how to avoid poorly structured code, and how program flow actually works.

The first revision took me around 6 months to implement, and the second about 3 days. Worth every moment of futzing around and hacking and designing.

The language is getting rewritten (this time with an actual grammar!), and I'm finding it still has a mountain of useful things to teach me about programming. I can't recommend it enough. :)