Foursquare

Foursquare is a website and mobile app that encourages its users to "check in" at points of interest by geographical location, and add additional points to the system if they are not already noted. This is done in a "gamified" manner with users getting points for their checkins (though these points don't actually get you any prizes or even recognition outside the circle of your friends who can see where you stand relative to them and their other friends). Many see this as pointless, but the userbase (in addition to giving Foursquare all sorts of details about their whereabouts and travels, for who-knows-what use they and the marketers they likely sell data to may make) does end up building a comprehensive geographical database of things ranging from stores to tourist sites. This database, in turn, is accessible through an API, and many other sites and apps use it (even Instagram, which uses it rather than its owner Facebook's own geographic database).

Links

 * Foursquare
 * Foursquare developer API
 * Foursquare lets others play with its API