Here's the entire source code + helpful annotations for the original Metroid game for the NES.
Labels corresponding to address values have been added to every line to make the code easier to follow for beginners interested in understanding the inner workings of a Nintendo game. The labels also make the code easier to debug if it is modified. At this time, the source code is still a work in progress but it is much farther along than the original document. The title page is completely documented. The intro routine, end routine, password scheme and sound engine are described in detail. About a third of the game engine is detailed and about half of each game area page.
(via @shauninman)