DesktopEarth

DesktopEarth is a very singular program, with a single purpose: it generates wallpapers for your desktop (in fact, the software applies the wallpapers automatically).

The images generated are a very accurate view of the earth from the space, at the precise moment of the update. You can see the sunlight projection over the earth, like you do in Google Earth: you can choose to get the current daylight image, or pick a specific timezone. For the dark part of the map, you can choose several textures to show, I liked to use the "Abundant City Lights", and there is a funny one, "Moonlit Surface"...it resembles the moon very well! ;)
But there is more: the wallpaper changes according to the season, so you will see major variations from summer to winter (ex. the snow cover and the foliage changes).
And to get it better, you can see the current cloud disposition over the earth, so the final result is just stunning.
There is also an online version from the author's site that enables the user to get images up to 1920x1200 in lossless PNG format. This is particularly useful if you are not a Windows user!