I have attended a speech at the local Linux Day 2013 about Mifare Ultralight NFC ("Near Field Communication") cards and implementation bugs that render the system vulnerable to some "attacks" (now fixed, too late ;-) ). They were at DEFCON 2013 too.
These cards operate at 13.56 MHz and can carry some data as well. They are being widely adopted as electronic tranportation tickets, access badges, ... Since they come with a factory programmed unique ID, some DIY recycle is possible at almost no cost.
The speaker has described the data memory map of those cards and explained how some bytes are one-time programmable: once a bit is set to "1" it cannot be returned to "0". That is how some transportation systems keep count of used/available tickets on the card. The data area remains rewritable and the unique ID cannot be changed.
Reading (writing) these NFC tags is pretty cheap (about 20 USD@2013) and can be done with a computer or an Arduino(-like) board. Of course some sort of software is needed to interact with the reader hardware and the card.
Since I am curious about this technology I have ordered a card reader/writer at least to see what kind of home applications can be tought of. A poor man's presence detection, for example, or an authorization system for you-never-know-what.
It is QRP afterall :-)