I did some development with Mifare Classic (ISO 14443A) cards about 7-8 years ago. You can read and write to all sectors of the card, IIRC the only data you can't change is the serial number.
Back then we used a proprietary library from Philips Semiconductors. The command interface to the card was quite alike the ISO 7816-4 (used with standard Smart Cards).
I'd recomment that you look at the OpenPCD platform if you are into development.
This is also of interest regarding the cryptographic functions in some RFID cards.
