Doors and the keys that open them have always been fodder for the creatively-inclined. Lockpicking and its associated hacks is a fairly common geek hobby, and people are frequently embracing more high-tech methods for opening their entryways, including using their smartphones for automated access or touchscreen entrypads.
And entering the realm of the remarkable, in 2009 a senior-year MIT student furnished his dorm room door with a hydraulic door opener and unlocker, all controlled through his iPhone. Chris Varenhor outfitted the door with a hydraulic actuator and valves connected to the dorm room sink; the movement of the hydraulics is controlled by the movement of the water in the valve.
On the back end, the Ubuntu machine controls for the "iDoor" are interfaced through a Phidget control board. Throw in some electronics, a power source, some switches and servos, and you've got the iDoor.
It's all controlled with a custom app on his iPhone, though it can also be activated by a series of knocks, detected by a vibration sensor at the top of the door.
I'm not sure about you, but if I'd tried this in my college dorm, the RA would have come a'knockin' pretty quickly, which is a shame. But now that we're in a place of our own, the world is full of possibilities for customization. As someone pointed out in the comments in Varenhor's original post, this type of setup could be very useful if you need to let friends into your house while you're away. What are some other uses for replacing a physical key with a device-controlled opener?