If you're inspired to create your own MP3 player but are either too cheap or too clumsy (in my case, both) to use Make Magazine's Daisy MP3 player kit, South Korea has the solution. The MOTZ DIY Music Box is a coin-sized, flash-based MP3 player kit that sells for $40 (U.S.). Once you've got it, you can join the corporate bandwagon of cramming an MP3 player into just about anything. The tough part will be finding something ridiculous that someone hasn't already tried putting an MP3 player in. The MOTZ is USB 2.0 by way of a USB-to-mini-jack connection (similar to the MobiBlu Cube). It has a Li-Polymer battery, 256MB of memory, and plays MP3, WMA, and OGG files.
For the uninspired, MOTZ also offers a prefabricated wooden case you can install the player into.