The proxmark3 is a powerful general purpose RFID tool, the size of a deck of cards, designed to snoop, listen and emulate everything from Low Frequency (125kHz) to High Frequency (13.56MHz) tags.
This device can do almost anything involving almost any kind of low ( 125 kHz) or high ( 13.56 MHz) frequency RFID tag. It can act as a reader. It can eavesdrop on a transaction between another reader and a tag. It can analyse the signal received over the air more closely, for example to perform an attack in which we derive information from the tag’s instantaneous power consumption. It can pretend to be a tag itself. It is also capable of some less obviously useful operations that might come in handy for development work.