RFIDuino is a Arduino shield which can read and write 13.56 MHz ISO-14443A (Mifare) RFID tags. It plugs upside-down on top of a Arduino, maintaining a 20mm board to board spacing. Mounting hardware and assembly instructions are included.
Free Arduino code library available soon. Here's a simple example to read tag ids.
The RFDIduino kit is now available at shop.marcboon.com.