Using an ATtiny85 as an RFID Chip
A simple an inexpensive way to emulate HID and standard 125kHz RFID cards by just using a microcontroller, an optional capacitor, and a coil:
http://scanwidget.livejournal.com/32928.html
Quite a cool hack, especially since it can emulate the FSK based HID cards (think building access control).
