Google Earth is an amazing virtual globe that allows you to navigate the entire planet in 3D graphics.  You can view any part of the Earth from any perspective or angle, from a vantage point deep in space, or as near to the ground as 100 metres.  In a matter of seconds you can zoom in from a continent-wide image to see individual landmarks in a city.

Since there is a completely free version and since it is such a great thing there's no excuse for not getting it!