I've had an Innovations ID12 around for a while that I wasn't using because the pins on the ID12 didn't work on a breadboard or connect to anything I have on hand. Sparkfun has a breakout board and special 2mm headers that are supposed to get it to work on a breadboard, but I never got that far with it.
I recently got the Sparkfun RFID USB Reader board to use the ID12 with a computer. It works fine as such. The board also has connections broken out so that it can be used on a breadboard and connected to an Arduino via a serial connection. Here is a brief example using the Arduino SoftwareSerial library. It reads a 125 kHz card and prints the card's number to the serial console when a suitable card is brought near.
Reader Board Arduino Uno
TX Digital 10
SoftwareSerial rfidSerial(10, 11); // RX, TX (TX not used)
Note that the board, which Sparkfun unhelpfully calls an "RFID USB Reader," does not actually include the RFID reader chip. In terms of cost, it's certainly cheaper to get the breakout board and solder the chip correctly, if you only need to use it with an Arduino, but the RFID USB reader board offer the flexibility of using the ID12 RFID reader with either a computer (via USB) or an Arduino (via TTL serial).