I live in residence at the University of Toronto, and in my building we have to use key cards to enter and exit our rooms. Like many students around me, I often forget my key card inside my room when I leave, locking me outside! EDIT: there is a youtube demonstration athttp://www.youtube.com/watch?v=2v30EHG4x_s
So, I decided to design a solution- an electronic door-opener which pulls down the door-handle of my door, allowing me to enter without my key.
If you have a similar situation, you might want to do something like this, but this was more of a fun weekend hack than a real attempt at an implementation, hence the huge quantities of duct tape.