This is an old, abandoned project. If you're looking for something that works now, try MP3elf.
A number of people are interested in using Dallas Semiconductor's TINI as the core of a networked MP3 player. This website collects together useful information, and free designs for players.
There used to be a mailing list associated with this web site, but it's no longer in use. If you're interested, you can look through the list archives for useful information.
I am working on a design for an MP3 player. Although I intend to use it as a piece of hi-fi equipment, with a display and user interface, the design can easily be adapted for use in other situations. The design uses the TINI for control, and to shift MP3 data from a TCP connection into a FIFO. An Altera FPGA provides glue logic, and on demand from the STA013 MP3 decoder reads from the FIFO and outputs synchronous serial data. The MP3 decoder outputs serial digital audio, which is converted to analogue stereo by a CS4334 DAC.
I am currently designing a printed circuit board for the project. The circuit board will include support for other devices, like additional serial ports - these positions do not have to be populated if not required. The circuit board should end up being around 120x180mm - slightly larger than Eurocard size.
I should be able to provide more information on the design given time. In the meantime, you can have a look at the work in progress.
This list is UK-oriented at the moment.