There were 2 projects back in the university that made me improve my programming skills: building an OS (multithreading based on a DOS system) and building a co

There were 2 projects back in the university that made me improve my programming skills: building an OS (multithreading based on a DOS system) and building a compiler (using JavaCC). In a different way, learning Lisp and Prolog, which basically widen your perspective. Also, I have a question; most people say that they learn by reading good code, which I agree. Do you have any recommendation of a piece of code that you consider it has "something to teach"?