RFID tags are all over the place. They're used in building access control systems, passports, inventory tracking . . . This instructable will show how you can use an Arduino and a few simple components (wire coil, transistor, capacitor, resistor) to make a device that can spoof an 125 KHz (low frequency) RFID tag. This is version 1, so there are many enhancements that can be made, but this version is stupid simple, yet it works. I did this in a few hours without much previous knowledge of RFID and without any fancy equipment (like a radio tuning hardware or an oscilloscope . . .I guess an oscilloscope is fancy, I need to pick up one of those).
UPDATE: Here is a link to an Arduino Mini shield based on these instructions http://wiki.smallroom.net/doku.php?id=terd:projects:rfidspoofer