Desktop Earth is an oldie, but a real goodie. It allowed you to display a globe or map on your desktop that would move according to the time, either keeping your location centered with light changing to show the relative position of the sun, or showing from the sun's perspective with the earth rotating or moving by.

While very cool, Desktop Earth is extremely long in the tooth. Luckily, a new version has been released, dubbed appropriately Desktop Earth 2.0. In the new version, the imagery is based on NASA's Blue Marble Next and Earth's City Lights, and is generated from very high-resolution textures (2560x1280).