OpenPCD has been doing a lot of work with RFID hacking. They have a bootable Linux distro available to make exploration easier:
The bootable Live RFID Hacking System contains a ready-to-use set of hacking tools for breaking and analyzing MIFARE Classic RFID cards and other well known card formats. It is built around PCSC-lite, the CCID free software driver and libnfc that gives you access to some of the most common RFID readers. See our tutorial video for a quick introduction on how to break MIFARE Classic RFID card keys using our Live RFID Hacking System.
This ISO (663 MB, for 64 bit x86 hardware only) is based on Fedora Linux and is designed to work with many commercial RFID readers. (It appears that the common Parallax RFID readers are not supported.)
Note that no expensive hardware is needed: it is also designed to work with the above simple homebrew LF RFID sniffer frontend for USB soundcards. Visit OpenPCD’s RFID page for the ISO, schematic and all the docs and links.
You can view the video tutorial here.