Open Electronics have introduced (originally by Andrea Fainozzi) an RFID shield which can be used for fun or for serious applications. It is based on ID-12 Innovations recognizer of passive transponders. RFID shield also contains buzzer to activate sound alerts and relay which may be used to activate door locks or other power devices.
No matter how good hardware it is useless without proper software that does all the tricks. They provide couple great Arduino examples where simple control using relay is implemented. Another example pushes the limits towards network. For this you will need a network shield where you will be able to log tag reads in to Google spreadsheet.