Variations of MIFARE Ultralight cards are becoming quite widespread. For example, huge transport companies employ them as payment cards.
As I get from MIFARE site and Wikipedia there's a full-blown microcomputer inside the card - with a processor, memory, etc. When I move the card close to the reader its field induces current in the card antenna, the card computer turns on and runs some complicated protocol to communicate with the reader.
That's all cool but could I write my own program that would run on such a card and do something useful a card doesn't do by default? What are simple examples of what such program could do?