Complete plans for PIC based mp3 player
Build your own player and learn how to interface with SD flash memory.
Obviously, building your own mp3 player is generally not cost effective if your only desire is to have a working player.
However, for the nerd wanting to build their own while learning to use a Microchip PIC microcontroller, the Daisy mp3 player fits the bill. The site features the complete information needed to build the player including the firmware, PCB files (Eagle format), manual, and instructions.
The project is neat in itself, but personally, I think it's a great source for example code and wiring which allows you use a PIC micocontroller to store/retrieve data from a common, cheap SD/MMC flash card. With this knowledge, projects can be built which feature low-cost extended data logging, sound recording, sound playback (for a home-build pinball machine for example 
) and anything else where the low-cost, non-volatile storage properties of flash are handy.
You can find the various files to build the Daisy released under the GPL for your hacking pleasure.
Related Stuff:
PIC microcontroller based video game systemUsing a trio of PIC microcontrollers, this guy built a fully functional video game which uses analog paddles and has...
Web server that fits into your pocket
An extremely small, full function embedded network server. Olimex has created a fully functional TCP/IP embedded server which they claim is...
Build your own home CNC machine to help with projects
Now you can use rapid-prototyping techniques on your personal projects cheaply. Anyone who builds their own hobby projects is probably familiar...
Simple DIY microcontroller based video game console
This open-source hardware and software project shows just how much can be done with modern microcontrollers. It implements a...
Build your own FPGA based Amiga 500 clone
The Amiga 500 was one of the very popular "non-IBM" choices back in the day thanks to its great selection...