If your home or office has unique security challenges or you just want the geek cred you assemble a system to open a door via an RFID signal. At around $US100 the parts aren't expensive, but in addition to basic electronics skills you'll need familiarity with Arduino Uno boards and access to a laser cutter.
MIT Researcher Valentin Heun created the RFID door opener as he was annoyed that he needed both his MIT ID card and a key to gain access to the MIT Media Lab. With this system his RFID-embedded ID card is read by an RFID evaluation shield connected to an Arduino Uno board. His RFID code is approved and a servo opens the door. A laser cut mounting board allows the servo and RFID evaluation shield to be attached to the door's handle and deadbolt.
See Valentine's site below for a ZIP file containing a full parts list, instructions and the template for the laser-cut mounting board.