Desktop Earth automatically generates a gorgeous desktop image of the Earth as it would be seen from space, at that moment in time, and then updates it every few minutes.
This starts with a high definition base image of the planet, updated monthly to give a more accurate view of snow coverage, the seasons and so on.
The picture then gets overlaid with cloud images, which can be updated regularly. These look gorgeous and give a realistic effect, but aren't actual snapshots, so don't rely on them for weather forecasting.
To finish, you can actually see the passage of daylight across the globe, so your desktop will very graphically show you where it's daytime right now, and where it's the middle of the night (it can even add city lights for a little extra realism).
And Desktop Earth will by default update the image every five minutes, to keep it reasonably accurate. (Although you can change the update interval and many other program settings from its system tray icon.)
If your desktop is covered in shortcuts, of course, the program may not be for you. They'll be more difficult to see, with the changing wallpaper (plus they'll obscure so much of the amazing visuals that you may wonder why you bothered).
If your desktop is normally clear, though, Desktop Earth is a great way to give it some life. The program is entirely free, no adware issues to worry about. And while it runs in the background, RAM use is minimal, at only around 5MB on our test system.