I think one of the coolest applications that Google has ever made is Google Earth. The application combines satellite imagery, 3D graphics, and the contributions of picture takers, to provide beautiful tours of locations around the around, right on your computer screen. A mobile version of Google Earth is now available for mobile phones running Android 2.1 or greater, which for now is the Google Nexus One. The application is in the Android Market, but only appears on the Nexus One. Google says that a version will also be available for the Motorola Droid. A version for the iPhone has been available for some time, and you will find it in the iTunes App Store.
The mobile version of Google Earth displays the same 3D imagery that is available on the desktop version. It takes advantage of the GPS in the phone to fly in to your current location, and perform a local search. You can use mult-touch to pan, zoom, and tilt your view in the application, and you can perform searches using voice input.