Proxmark3, originally developed by Jonathan Westhues and release under the terms of GPL.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 analyze 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.
This Proxmark3 kit includes everything (in fact more than) you need to get started doing RFID/NFC research in style.