Google Latitude: Tell Your Friends Where You Are
Using a combination of Global Positioning System, WiFi, and cell tower location data, the service, an extension of Google Maps, Latitude can determine where you are in the world via your mobile device depending on which of those technologies the device can use. It will work on most color Blackberries, most Windows Mobile 5.0 devices, most Symbian S60 devices, and phones powered by Google's Android mobile software, such as the T-Mobile G1. No iPhone or iPod Touch yet, but Google says that's coming very soon.
You download a new version of Google Maps onto your mobile device, then invite friends via an email to join. Once they accept, and if they have a Google account with a profile picture, you'll see that on a map. Then you can send them an email or text message or call them, and of course get directions to their location. You can also view locations of your friends from your desktop through iGoogle, the company's personalized home page.