The Play-Yan was an accessory for the Nintendo DS as well as Gameboy Advance that allowed you to play MP3 songs. It plugged in an SD card into a GBA cart, and plugged in on any system that could play GBA games.

Though it flopped in Japan, at the time I really wanted one. MP3 players at the time were pretty expensive, so this seemed like a bargain to what I heard it cost back in the land of the rising sun.