E-Reader

http://images.tomshardware.com/2005/12/10/nintendos_ups_and_downs/ereader.jpg

A peripheral released for the Game Boy Advance/SP in 2002 that unlocks special content for certain games. Content in cludes mini-games, different worlds, or full and complete NES games. The content was added or unlocked by purchasing e-Reader cards and swiping them in.