Did you know that on Super Mario Advance 4: Super Mario Bros. 3 for the GBA you could upload extra content including up to 32 new stages per game with the e-Reader? Some of the types of cards in it were the demo card; showing tips and tricks by the developers, the power-up cards; which allow new power-ups to be added to the game, like the cape feather, and the switch card; which allows players to add projectiles from Super Mario Bros. 2 and do some other things. There were also level cards which allowed you to upload levels.
Resources:
http://en.wikipedia.org/wiki/Super_Mario...io_Bros._3
http://www.mariowiki.com/Super_Mario_Advance_4_e-Cards
http://www.mariowiki.com/Super_Mario_Bro...io_Bros._3