The D3 meteorite map project maps all of the meteorite impacts on a map of the world. The size of the circle on the map corresponds to the mass of the impacting meteorite.
When I first started with D3, this was the type of thing I really wanted to create and this was, along with the force directed graph, my favorite of the D3 projects. Using a bunch of D3 paths to draw a map is almost like magic.
I used a video tutorial by Jonathan Soma to help me use geoJSON and build maps for the first time.