There are all kinds of keyless door locks on the market these days, but this DIY version utilises a touch-sensitive system powered by an arduino that offers a cheap, secure and discreet method of entering a locked room. To gain access, the user must touch a wire jutting out from the door in a specific sequence. Yeah, this project is super-ghetto, but it would be awesome integrated into the doorknob itself.
Apparently, the problem with that was that the knob had too much surface area for the sensor—but I don't see why the entire doorknob needs to be utilised. It would be even more secure if only touching a specific section would grant access. If you want to try it yourself, the complete instructions are available on the project page. [Washington.edu via Hack-a-Day]