I've recently been working on a project that utilizes the SM125 RFID module from SonMicro. As part of that project I made this library that implements all of the publicly described functions in the SM125 datasheet. There is some functionally mentioned in the datasheet that unfortunately isn't described well enough for me to implement it yet.
The library allows reading EM4102 cards and reading/writing Q5 and Atmel T55x7 125khz cards. I have done quite a bit of testing, but as this is my first Arduino library, its very possible I missed something.
The code is available at GitHub: https://github.com/HarvestGardener/SM125-125khz-RFID-Arduino-Library
I hope it is useful to someone, somewhere.