Graphical depictions of the code and behaviors in Atari 2600 games by Ben Fry. Above: Detail from Pitfall. Below: Pitfall's complete code rendered.
From his description of the "Distellamap" project:A detail from Pac-Man:
Like any other game console, Atari 2600 cartridges contained executable code also commingled with data. This lists the code as columns of assembly language. Most of it is math or conditional statements (if x is true, go to y), so each time there's "go to" a curve is drawn from that point to its destination.
Fry's website includes renderings of the code from Adventure, Air Raid, Combat, and several other games, and more about his process. [Thanks, Matthew!]