Mapping all the rivers in the United States
Inspired by Ben Fry's All Streets map, which showed every road in the United States, Nelson Minar mapped every river to similar effect. As you'd expect, the geography of the United States emerges without actually mapping locations.
We saw a similar map from National Geographic, which showed the rivers of the world and took home an award for best map of 2010 at Malofiej. So Minar's map isn't especially new, but the good bit is that Minar posted a tutorial and his code on github, so that you can see how such a map is made.