I built a RFID cardreader that uploads each scanned card's unique ID to a MySQL database. This system will be used in our FabLab for membership and reservation management.
I used an Arduino Deumilanove, an Ethernet Shield, a SonMicro SM130 RFID reader module + antenna and a bunch of wires, and components.. The finished project is mounted in a custom laser cut enclosure so that it can be used on the counter next to the cash register.
More pictures and a couple of movies can be found at my Flickr photostream:
The box runs standalone, the USB connector is only used for power and viewing serial debugging info.. It could work anywhere, as long as it has a power source and an internet connection.
The php/mysql part is far from done, I'm still working on it.. but any good php site will have dozens of examples where posted forms are stored in a database.
I got the rfid stuff from SonMicro, but Marc Boon started selling awesome kits to make RFIDuino shields: