http://veimages.gsfc.nasa.gov/1438/earth_lights_lrg.jpg

This is a NASA image of the world at night, I'm guessing since you can't take the picture in one go, it's probably super-imposed a few times.

I love how the busy cities/places light up!  (ha I made a pun!)

It's a very nice picture for desktop background too -  it's a bit big and if you use the Stretch option it should fit any desktop size.