A new idea for a more advanced product: the RFID cloner.
It would have the same form factor as the RFID sniffer, but instead of merely detecting presence, this device can read the unique serial number of a RFID tag, store it at the press of a button, and subsequently imitate a tag using that same id. (which is then not unique anymore!)
Basically it would be a hybrid device, consisting of a RFID reader and a (active) RFID tag, both using the same antenna, and running from a battery. It could also have a USB port for sending the id to a host computer, setting the tag id, and charging the battery.
I wonder if anyone did this before...