I just purchased the RFIDRW-E-USB RFID reader from Priority 1 Design (see this page, datasheet available in PDF form.)
I have tried to make it work on my computer (Ubuntu), before plugging it to my Raspberry Pi, but I had not luck so far and I am running out of ideas on how to make it work.
I connected the provided antenna to the A+ and A- nodes on the board but did not solder it in place, as I plan on installing another antenna later and I'm not really good at welding. (I made sure the stripped part of the wire was touching a metal part on the board, and taped them in place.)
Then, I plugged the whole thing in my computer, and I can see the board as /dev/ttyUSB0.
Then, inspired by the code for a Parallax RFID reader (from here), I tried the following in Python (with super user privilege):