Foursquare is a location based social networking service allowing users to "check-in" to specific locations using their smartphones. Foursquare is a game; you earn points for "checking-in" and the person with the most points at a specific location becomes the mayor of that spot. However simple it sounds, Foursquare is quickly rising in popularity and provides users a new way to enhance their social network.
Foursquare also provides businesses the advantage of free advertising. Restaurants are now giving away promotions every time someone "checks-in".
Check it out! Foursquare