London's Transport Museum commissioned a gorgeous interactive map called The Real Underground which lets you see two of the many versions of the world-famous London Underground Map, as well as a street map, and switch among them to see how the two schematics agree with actual geography. I wasted way too much time playing with the Real Underground. Now it's your turn. (Looking at the evolution of transit maps, you can see that it was Harry Beck's breakthrough map from 1933 that changed the face of tube maps forever.)

There appears to be a similar effort to do the same to the New York Subway map, published by the brilliant folks at Urban Mapping. I visit New York only once every other year or so, but this map is so cool I'm tempted to get it anyway, even though I'll probably misplace it by the time I next travel there.