Via TreeHugger, I found this cool work by artist Aaron Koblin:

Created by artist Aaron Koblin. See full-sized version and other versions.
This beautiful visualization is part of his project “Flight Patterns”, showing the density and path of airplane flights over the United States. The project features several different visualizations in different styles, but I’m partial to blue. He also created several animations that add a whole new level; it’s especially neat to see the wave of increased air traffic travel east to west as morning arrives in each time zone.
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Comment by Airplane Flight Pattern Graphic « Greg Laden's Blog — 3 December 2007 @ 06:34 UTC