Google Earth is one of the most recent desktop applications from Google. And like other products of this company, it's really a revolution.
A few years ago (let's say, 10 years), we enjoyed some geographic encyclopedia products like "PC Globe", in which we were able to see and print some simple maps, a brief information, and even hear the national anthems of countries with the PC speaker!
Later on, computers evolved, the graphics and sound in applications, too. Newer encyclopedias like "Encarta" showed us more possibilities, adding sound, video, some panoramic points and hotspots with miscellaneous information.
But now, things are really different. Now, you can have a simple application that connects to a huge maps database on the Internet, to retrieve images and some information from any point of this planet. It's like a virtual reality tour, a fresh new experience. You can navigate around the globe with a very clean user interface, rotating, panning and zooming the maps. So, one moment you are seeing the Washington Capitol Building, and a few clicks (rotations and zooming) after, you can jump to the Kingdom of Butan in the Himalayas Mountains.
Of course, there are not just maps here. You got a layered database with 3D buildings, streets, several divisions, places of interest (linked with HTML content), search capabilities, an internal browser, and a lot more.
It is for free, so what are you waiting for?