Designed in cooperation with Grand Idea Studio, the Parallax Radio Frequency Identification (RFID) Reader Module is the first low-cost solution to read passive RFID transponder tags. The RFID Reader Module can be used in a wide variety of hobbyist and commercial applications, including access control, automatic identification, robotics, navigation, inventory tracking, payment systems, and car immobilization.
There are a variety of transponder tags that come in different packages. Each tag has a specific range that is within 10% of the given distance for each type of tag. The reason for the 10% is due to environmental conditions and RFID modules.
Features:
- Low-cost method for reading passive RFID EM4100 family transponder tags
- 2400 baud serial interface
- Enable input allows module to be enabled/disabled by software
- Bi-color LED for visual indication of status
Compatible tags:
Key Specifications:
- Power requirements: 4.5 to 5.5 VDC
- Communication: Serial at 2400 baud (8N1)
- Dimensions: 2.45 x 6.25 x 0.22 in (62.2 x 82.5 x 5.57 mm)
- Operating temp range: -40 to +185 °F (-40 to +85 °C)

Note: No tags are included with the RFID Reader Module. This device can be connected to a PC serial port using a MAX232 line driver. The circuit isn't supported by Parallax, but it's possible to make this connection with a few dollars of parts or by using the USB version of the reader. There is an outline of connections on the forums.
Parallax Perspective:

Click to Watch Video
Downloads & Resources:
RFID Reader Software on Forums
RFID and Text-to-Speech Modules Video Podcast
RFID Reader Documentation v2.1 (.pdf)
BASIC Stamp 2 Source Code (.BS2)
RFID Basic (.BS2)
BASIC Stamp 1 Source Code (.BS1)
SX/B RFID Card Reader Code (.SXB)
RFID Security (.SXB)
RFID Door Lock Video (.mpg)
N&V column "You Can't Touch That" #120 (.pdf)
Nuts & Volts Column RFID (.pdf)
EDN Article: RFID in Embedded Designs: Your Move (Off Site)