SPI
https://github.com/miguelbalboa/rfid
http://hetpro-store.com/TUTORIALES/modulo-lector-rfid-rc522-rf-con-arduino/
8 €
http://www.dx.com/p/nfc-shield-rfid-rc522-rf-ic-card-sensor-module-s50-rfid-smart-card-for-arduino-uno-mega2560-383996#.V50G_X3cDMI