Rifidi is a middleware and development platform for RFID. With Rifidi you can prototype an RFID application down to the core RFID Hardware and using Rifidi Edge you can easily develop business rules to turn RFID events into real business applications.
NFCProxy is a an Android app that lets you proxy transactions between an RFID credit card and a reader. The saved transactions can be replayed to skim credit cards or the RFID credit card can be replayed at a POS terminal.
NFCProxy can also be useful for protocol analysis to learn about the underlying communication protocol.
In order to replay credit cards to a reader you MUST install a special version of CyanogenMod. See Wiki.
Slides from the Defcon 20 presentation of NFCproxy can be found at: http://blackwinghq.com/labs/presentations/
Max Microchip GPL is a multi-protocol RFID reader for scanning pets implanted with an under-skin ID microchip. The external hardware required can be built with no soldering, and connects simply to the PC's Line In Audio jack.