There's a homemade MP3 player called the Minty, it fits in an Altoids tin and uses USB to download songs.

Here's the forums discussing how it works, how to build it, what chips to use, etc:
http://www.ladyada.net/forums/index.php?sid=360e63573598b3933e1aef60c1c5acfa

Here's the Minty site with scehmatics, pictures, etc:
http://www.ladyada.net/make/minty/index.html

Here's the MiniMinty site, a variant that fits in an Altoids gum tin, with schematics, pictures, etc:
http://industrialogic.com/mini-minty/index.html

Hope that helps some.