Foursquare

Foursquare is a mobile application that makes cities easier to use and more interesting to explore. It is a friend-finder, a social city guide and a game that challenges users to experience new things, and rewards them for doing so. Foursquare lets users "check in" to a place when they're there, tell friends where they are and track the history of where they've been and who they've been there with.

See the reference side-by-side comparison Foursquare vs Facebook Places, Google Latitude, Gowalla, Loopt