Google Latitude App lets you share your location with friends.
You can also track your friends’ location on Google Maps. This was released for iPhone, iPad and iPod Touch recently and is available at the Apple Store for free. You can also toggle background updating and/or just choose to share the residing place.
I am not sure why people would like to share their location continously! Its a waste of battery and security issue 🙂
Latitude is available for Android, iPhone and iPod touch, BlackBerry, Windows Mobile, and Symbian S60