EarthDesk is a Mac OS X application published by Time Palette Software that replaces your desktop image with a map of the earth that can be updated in close to realtime.
This doesn't seem like such a great thing to have, particularly at the price (it's $19.95 and distributed as shareware) but I was greatly impressed by the wealth of features included in the product. Your map is extremely customizable - you can pick where to center it, either by choosing from a long list of preferred cities, by inputing map coordinates or by electing to follow the sun- or moonrises instead (so that the Earth rotates instead of day and night coming and going on one geographical location).
You can change the kind of map displayed as well, selecting from an inclusive list of flat maps, mercator-like maps and globes, as well as tweaking its magnification settings (from 50% to 400%) and update frequency (from every minute to every fifteen). Even the moon is configurable, allowing for continuous full moon, no moon or natural moon shadow. If you prefer your map to look more, well, map-like, the image can be displayed with political characteristics instead of geographical ones. City lights during night cycles are featured in all maps except the political one.
Earthdesk can also pull location information directly from GPS if one is so equipped.
It's probably worth noting as well that it doesn't seem to be much of a resource hog - it only uses memory when swapping images, and then practically none for a fraction of a second.
I'm not sure it's worth $20, but the program IS beautiful to look at and is a nice change from a static desktop.