[pc486] sent in his hack that uses an ATtiny85 to act like an EM4102, a chip used for RFID tags. Minimally, all that is needed is the AVR and a coil, but he recommends some filtering capacitors.

Applications of radio-frequency identification (RFID) are widespread in today's well-connected society. Found in nearly every market including government, transportation, food, health care, retail, ...