icbank.com, a technology firm in Korea makes these cool MP3 kits you can build.
They have two different types of MP3 players that you can build.
One of them uses a hard drive and the other one uses an SD memory card as storage.
The whole thing runs on a Atmel ATmega 128L, which is kinda cool.
They also give you the source code to the firmware, so you can customize after building it.