If random points on a plane are post offices, then Voronoi Diagrams divide the plane into the most efficient zip codes.
I found a Voronoi diagram on the internet. I duplicate its calculation here using Photoshop's 'Lighten' blend mode and some radial grads.
How to: Draw the Voronoi Diagram
Weather Sealed: Where the Buffalo Roam
This Voronoi Diagram traveling matte effect is pretty cool.