Project:RFID Reader Writer
From London Hackspace Wiki
You can find the RFID module on ebay around £12.50 - £14
I bought the one below but it looks like it run out of stocks
http://www.ebay.co.uk/itm/RFID-13-56Mhz-Module-Tags-SPI-Interface-Read-Write-for-Arduino-UNO-R3-Mega-MCU-/170859728113?hash=item27c8088cf1
Source Code : http://www.rcubestation.com/drivers/rfidcardset_sample.zip
The one below is the same from another vendor
http://www.ebay.co.uk/itm/RFID-module-Kit-13-56-Mhz-with-Tags-SPI-Write-and-Read-For-Arduino-/200828954356?pt=UK_Sound_Vision_Other&hash=item2ec256a6f4