Location based social networking website. Users with GPS mobile devices can check-in at venues and award badges.
Foursquare is a location based social networking website. Users with GPS-enabled mobile devices can run the Foursquare app (available for a variety of devices such as iPhone, Android, Blackberry, etc) to "check-in" at venues to award points and badges.
The Foursquare API gives developers access to all of the data used by the Foursquare mobile applications, and, in some cases, even more. Over 30 million people worldwide use Foursquare, accounting for more than 3 billion check-ins, with millions more each day. And more than a million businesses use the Merchant Platform.
Resources
API Documentation: https://developer.foursquare.com