Now go outside and look at the sky.
This must be one of the best applications ever to be released for free... Google Earth
It's a free PC download and it literally brings the world to your desktop. The basic concept is very similiar to Nasa's World Wind, but in addition to a smooth 3D-rendered Earth, you also get the muscle of Google's search engine.
It overlays the satellite maps of this world's cities with icons for restaurants, cafes and shops and it provides driving directions courtesy of Google Maps.
Now... a Mac version would be nice. But otherwise it's perfect.