Take a look at this very cool image of the Earth at night (thumbnail above)
http://incredimazing.com/page/Earth_at_Night-686

The different colors are different sources of light – red is wildfire, purple/yellow is city light, green is natural gas flares, and blue is fishing fleet lights.
The image cites NASA’s Visible Earth 2001 as the source of the map, and the Defense Meteorological Satelite Program as the source for the light data.