Google Latitude for Google Maps for mobile and iGoogle gadget allows you to share your location with your friends and to see their approximate locations, if they choose to share them with you. Google Talk is integrated with Latitude, so you and your friends can update your status messages and profile photos on the go and see what everyone is up to. You can also call, SMS, IM, or email each other within the app.
How to use it?
From your mobile smartphone browser, visit google.com/latitude to download the latest version of Google Maps for mobile with Latitude. Latitude is available on Blackberry, S60, and Windows Mobile, and will be available on Android in the next few days. It’ll be coming to the iPhone, through Google Mobile App, very soon.
On you computer: visit http://google.com/latitude to install the Latitude iGoogle gadget and share your location right from your computer. If you have Google Gears installed in your browser (you do by default if you use Google Chrome), you can automatically share your location; otherwise, manually set your location to let your friends know where you are.
More info: Google LatLong Blog