Foursquare is an online service that lets you share your current geographic location with your friends using your mobile phone or even a desktop web browser.
If you are in the US, you can also update your location on Foursquare through a text message. You can use foursquare anywhere in the world.
The foursquare apps for iPhone, Blackberry, Android and Palm use GPS to show a list of nearby places. If you’ve been to a place more than anyone else, you are known as the "The Mayor" of that place.
Also see: Location Tracking with Google Latitude