One of the fascinating parts of old-style scrolling 2D video games was that you were playing in a large world, but only seeing a tiny part at a time (the part that fit on the screen). Using screen captures from game emulators, Ian Albert has produced huge single images of the worlds you were exploring piecemeal: Video Game Maps. An impressive piece of work.

From Ian's FAQ, there are also VGMaps.com, with many more visual maps, and GameFAQs, with text walkthroughs.