RFID is an interesting technology with a lot of potential uses for Arduino projects. Parallax RFID tag readers are a great RFID solution for use with Arduino because they're so simple to implement, plus they only need two pins to interface with Arduino. One pin activates the reader and the other transmits the code via serial connection. Radioshack has the Parallax brand readers on clearance. My local Radioshack actually gave me some for free because they couldn't find them in the system. Once I had a reader and tags, the first thing I did was use the software serial library to relay the RFID tags via serial to the computer. Receiving the tag via the serial monitor was interesting, but fairly limited in utility. I decided I wanted to relay the tag code as keyboard input so that it could be used for something such as a password, but that required the Arduino to act as a USB human interface device (HID).
I had a little trouble figuring out how to make the Arduino act as a keyboard so I decided to share what I've learned by posting an Arduino as HID device tutorial below. Check out the demo and tutorial below.