Oregon Scientific made the MP120 256MB credit card sized MP3 player with supposedly the world's first see-through display.
Not sure why a see-through display is useful on an MP3 Player, but it creates kind of a unique look. The Oregon Scientific MP3 player supports MP3, WAV and WMA formats, has an FM tuner and voice-recorder. This 256MB MP3 Player is really thin with 6mm. Overall it measures 90x53x6mm.
Gadget Store IwantOneOfThose sells the See-Through MP3 Player for about $212.85 (£129.00).