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