Google Earth is a 3D satellite view of the world that can be zoomed & rotated to display a full 3D representation of the landscape.
It features overlays for cities, boundaries, city search, city features (parks, roads, restaurants, etc), and more.
Ostensively the same as NASA's World Wind (cƒ) but only 10% of the size, Google Earth offers most of the same features and is, in some ways, a little easier to use.
However, it lacks many of the high definition maps, specially for locations outside of the US, and the overall map quality in some areas is very crude. Even within the same city you have a mix of high and low resolution maps (see Screenshot below) and it is frustrating to have the hi res map "run out" just when you need it most.

 

Google Earth can zoom in to under 3,000 feet (1 kilometer) above the ground and still get good clear images if hi res is available. Otherwise you will find that anything less than 30,000 feet (10 kilometres) above the ground will render the images blurry. If axis tilting is used, image quality generally deteriorates slightly.
You will need a reasonably modern computer and video card to view the images, and a broadband (128 kbps or faster) Internet connection to download the hi res images (downloaded automatically as you zoom in).
Your computer will have to be at least a Pentium III 500MHz (a 2.4GHz CPU is recommended) and your screen resolution should be, according to Google, 1280x1024 (32-bit true color). However, the program's window can be reduced to 800x430 pixels, so an 800x600 screen resolution will probably work OK. Your 3D graphics card should have at least 16MB of VRAM. Google reports that the following ATI cards are currently not supported: ATI Rage Mobility; ATI Xpert; & ATI 3D Rage.
Like NASA's World Wind, this is an impressive program but, in view of its much smaller size, we consider Google Earth to be the better value for your download dollar, specially if you are based in the US and tend to mainly view US images.