[paulmooreparks]

For me, it was implementing a large bit of the C standard library. I had just blown an interview at Microsoft in 2001, where I stumbled over a whiteboard implementation of strtol, and I thought that I needed a serious upgrade of my understanding of library implementations and the algorithms that are used in them.

Every developer should also implement all the basic data structures and algorithms -- and then never write their own again! The process, however, definitely improves your chops, helps you understand the trade-offs inherent in choosing among data structures and algorithms, and gives you an appreciation for what's going on "under the hood."

[paulmooreparks]

Also, writing a compiler covers a lot of the list above. Even a simple language like Brainfuck (my choice for this rite of passage) can teach you a lot. It also gets you into the world of assembly language, which it seems that fewer and fewer new devs are exposed to these days.

(Yep... I'm officially old now. I just played the "kids these days" card.)