The programmers at a web development company in Brooklyn, New York, altered the front door to their office so that it opens whenever someone checks into that location using Foursquare. Nick Hall, one of the owners of Apartm.net, explained how it works:

The key to the Foursquare door is a little Web relay device, which actually hosts its own little webpage (aw!) that the brothers use to run some Javascript. "The relay is supposed to be for industrial use," says Hall. "I think it's meant to be used to control pumps."[...]

A Mac Mini chills out near the door and makes requests to Foursquare's API every three seconds, looking for new check-ins at Apartm.net. When the computer finds one, it contacts the Web relay, which sents a simple binary bbzzzz! through a little copper cable that has been soldered to the intercom button in the hallway. The intercom is fooled into thinking the button has been pressed, and it unlocks the door via the building's existing buzzer system.

Link | Image: Foursquare