The SparkFun RFID USB Reader is a simple to use, USB to serial base unit for the ID-3LA, ID-12LA, and ID-20LA readers. Simply plug a reader into the headers and attach a miniUSB cable. Open a terminal program of your choice at 9600bps 8N1, then scan your 125kHz ID tag and the unique 32-bit ID will be shown on the screen. The unit is based on a FTDI chip and comes with a read LED and buzzer.
This new revision uses SMD headers for the RFID module, and has a solder jumper which allows you to disable or enable the buzzer.
Note: This product does not come with the RFID reader. Check below for compatible readers.
Resources

Schematic

Eagle Files

Tutorial

USB FTDI Drivers for Linux, MAC, and PC

GitHub

RFID Basics

FAQs:
Q: What RFID readers are compatible with the SparkFun RFID USB Adaptor?
A: It is compatible with ID-3LA, ID-12LA, and ID-20LA RFID readers.
Q: What type of connection does the adaptor use?
A: It uses a mini USB connection.
Q: Does the adaptor have any indicators for reading tags?
A: Yes, it includes a read LED and a buzzer for indication.
Q: What frequency of RFID tags can this adaptor read?
A: It reads 125kHz RFID tags.
Q: What is the output format of the RFID tag data?
A: The adaptor outputs a 32-bit ID via terminal programs.