Foursquare is a mobile application that is a cross between a friend-finder, a social city-guide, and a game that rewards you for doing interesting things. Foursquare lets you "check in" to a place when you're there, tell friends where you are, and track the history of where you've been--and who you've been there with. Foursquare challenges you to explore your city in new and fun ways by earning points, winning mayorships, and unlocking badges and specials for trying new places and revisiting old favorites.
From foursquare Official Website
Youtube video tutorial
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