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.
Voronoi Diagram by Paul Herron
UPDATE 9/16/2009
How to: Draw the Voronoi Diagram
UPDATE 9/27/2009
Visualizing the Distance to the Nearest McDonalds Restaurants
Weather Sealed: Where the Buffalo Roam
UPDATE 10/6/2009
This Voronoi Diagram traveling matte effect is pretty cool.
They also look a lot like bubble formations - with the radial gradients you almost get a 3D bubble effect...
The difference blend mode is really odd. They act like rubbery egg yolks when you pass them by each other.
That...is awesome.