Code to read

Assembly

Richard WM Jones's Forth - Richard WM Jones's literate x86 assembly implementation of Forth

C

Nethack - Single player dungeon exploration game
PostgreSQL - Object-relational database management system

C++

Stockfish - Chess engine

Common Lisp

cl-ppcre - Common Lisp regular expression library

Haskell

Scroll - Roguelike game developed during the 2015 7drl challenge. Joey Hess wrote a series of blog posts describing the development of the game.