Open source web check in api?

This might be very stupid question.. I was wondering开发者_Go百科 how google/facebook/foursquare web check-in work in technical aspect ? How do they get all the list of check in places? Would it be very difficult to come up with my own apps just like those web application? Or is there any open source web check api that I can use and develop my own personal app ? Thanks in advance !

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Look into the Google Places API. I think Gowalla uses it.

http://code.google.com/apis/maps/documentation/places/