RFID Reader

A 125Khz RFID Reader with a simple RS232 based ring network - to allow connection of multiple units to the same control computer.

Consider using "RS-232D", aka "EIA/TIA-561". It has all the same wires and voltages as RS-232, but specifies a RJ45 connector.

Also consider RS-485.

(What is the name of the standard for RS-485 on RJ14 ("4 wire RJ-11") or RJ45 ?) (Ethernet? =P)

Homemade RFID Reader which links to "Proximity Security System" by Craig Ross and Ricardo Goto which has detailed schematics and assembly notes for a RFID system with a $50 budget.
Adding a remote antenna to Parallax’s RFID reader
RFIDIOt: an open source python library for exploring RFID devices.
"ThingMagic Mercury4 RFID reader": "ThingMagic is shipping a Linux-powered RFID reader based on a software-defined radio (SDR). The company says the Mercury4 can read any RFID tag format, including those not yet invented". The TI 5502 DSP running "MercuryOS" runs the software-defined radio (SDR). The Intel IXP420 running Linux runs the web server hosting the remote configuration interface.
RFID enabled flame shooting trampoline !
Wireless Home Security Camera Reference Security For Home
RFID tribe Yahoo group
the OpenPCD Wiki: the OpenPCD Passive RFID Project is developing open hardware/"free hardware" for RFID.

further reading