The D3 force directed graph project shows national contiguity using a force directed graph. Essentially, if two countries touch on a map, they are connected on the graph with a line. You can grab a country's flag and drag it around the graph area and see the rest of the flags move in relation to the one you are dragging.
This was a really fun project cause it started to really show what D3 is capable of. Being able to drag things around in real time is really cool.
I used two resources to help me wrap my brain around forces in D3. A tutorial series by Stephen Thomas was very helpful along with this video tutorial by Ben Sullins.