If you're sick of using your boring old keys to unlock your door, Make has a guide for using an Arduino to power a lock that you can control with your smartphone.
The build uses an Arduino, a Bluetooth plugin, and a few chips to control the the process. The end result is a system where you can enter in a code on your phone that's transferred over Bluetooth then unlocks the door. It's not an easy build, but the guide walks you through each step carefully. Head over to Make for the full guide.