Half-Life, the first eight levels, by Corinthian.

[Updated: find high-resolution version right here!  Thanks, Corinthian!]

Tbh Half Life 1 is a fairly easy game to remember, as you really only go through one room at a time, and there are no loading screens to “disconnect” you. 

…Although I find it quite easy to remember what you get attacked by in each room, and even the order of the the rooms, it’s rather harder to remember whether empty corridors and vents turn left or right, or how many times. In some cases I have conveniently twisted an unmemorable corridor so that I could keep drawing in a direction where there wasn’t some map already…

[Josh says: holy cow.  This is epic.  Corinthian, you got a higher-resolution version of this available?  Let me know, tumblr’s image size limit makes the small text pretty much unreadable even zoomed in.]