I recently won a programming contest that netted me a gift card for ThinkGeek and not knowing what else to do I strolled the site looking for something interesting to use the gift card on. Eventually I ran into the RFID Experimentation Kit they have and decided that was what I needed. I have been wanting to play around with RFID for a while and this kit turned out to be pretty nice for tinkering.
Even though I picked this RFID kit from ThinkGeek you can buy it a couple different places and in a number of different configurations. The one I got came with a book from ExtremeTech called RFID Toys: 11 Cool Projects for Home, Office and Entertainment and the RFID parts are from Phidgets, who actually make a lot of other interesting kits.
This RFID kit reads EM4102 type tags and operates at 125 kHz. The frequency and tag type are actually important. I have a number of badges and keyfobs outside of what I got with the kit and none of them work with this reader.
The Phidgets site has source code in a number of different languages making it very easy to develop custom applications. To get started you will need to grab the driver source and compile that. It isn't a driver in the normal sense of the word since it doesn't get installed by your operating system but more of a base library. Once you have the base library compiled you can grab any number of different language source files from the RFID reader source area. There should be a simple RFID reader example that shows how to use most of the available functions out of the base library.
If you are interested RFID projects check out these links: