Elmue made his own RFID door lock with battery back up.  He was inspired after reading some articles about home burglars.

After searching for an electronic door lock, and not being satisfied by what he found, Elmue decided to build his own.  The design features a powerful battery back up, an easy to use interface, the ability to store up to 64 users, and it mounts inside the door so it’s not visible from outside or exposed to the elements.

There is an excellent, detailed write up on the project that include a schematic, parts list, and code.