Google Earth is a virtual globe, geographical program created by Google. The program uses digital elevation model (DEM) data collected by a NASA space shuttle to create the 3D map. With Google Earth, viewers can view satellite imagery of large portions of the globe. Users can also explore ocean topography, and switch to street view which enables someone to explore roads as if they were in a car. New features of Google Earth allow a user to explore regions of the moon, Mars, and constellations in the sky. New features are being added and existing features enhanced frequently.
Google Earth is available for free download here. A Professional version with enhanced geo-coding, map-making, and GIS features is available as well.