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/