The Earth is probably the best planet I've ever been to, so I like to keep abreast of what's going on in the world. I found this desktop background great for monitoring cloud cover, the advance of day and night and noctornal lights around the globe.
Although the cloud cover only renews itself every 3 hours, you might be able to find a server faster than those built into the Desktop Earth and apply this for quicker generation.
The illuminated surface of the Earth updates every minute though, so it's great for finding out where the Sun is concentrated at any given moment.
I would have liked to have seen a few more options, such as the ability to rotate the image on your desktop, but on the whole Desktop Earth provides a decent solar clock, a rudimentary weather chart and, more importantly, a stunning view from your desktop.