We used to see lots of MP3 projects where they had everything needed to play music. As the rule most of them were equipped with LCD, buttons and other user interfaces. Open Electronics have posted a bit different open hardware project where MP3 player were designed as module that can be used in different ways.
It is assembled on a compact PCB with 14 pin row . This makes it convenient to plug in to breadboard, other project or simply run as stand alone MP3 player with couple additional components. From technical point of view mp3 module is powered by PIC24 microcontroller. MP3 is decoded by VS1011 chip it already has a microSD socket that is able to accept up to 16GB of flash cards. Module can operate from 3.3V or 5V with integrated 3.3V regulator.