Jeremy Blum published his interesting idea of opening doors. Instead using locks or RFID swipe cards he suggests using QR-code based entry system. Reading QR codes requires camera. BeagleBone is powerful enough and is based on Linux so programming and interfacing is pretty easy.
Suggested system works as follows. No matter where this system is installed, users are registered within web interface. After user is approved he gets QR code via email that can be printed and used.
This system seems to be weak from attacks. QR codes can be photographed from distance. But for less secured areas might be a fun to use.