Since I plan to play with RFID, I've got a reader: "ID12 from Innovations" (reader and breakout board, and here's the datasheet). It can recognize standard 125 KHz RFID tags (like this other one), so can be used with tags easily available (e.g. on the bay). There are plenty of info (e.g. on Arduino Playground, on projects based on this reader), just search, ehm google "Arduino RFID ID-12".
The reader comes with a 2mm pin spacing:
so, adding 2mm heder pin to the breakout board:
and adding a more common header pin to the breakout board (with the help of a breadboard):
the module is ready to be used:
Here's a schema of how it can be connected to the Arduino board:
Once a TAG is recognized, the LED will light up for a while.
Planning to use the reader in a more stable way, here's a schematic of the realization of an interface board using a stripboard:
And here's a realization of that interface board:
The connectors for LED and for power and data have been salvaged from an old PC, so writings are odd: