Gio over at diyAudioProjects writes:

This is a small headphone amplifier that I built into a dead external CD-ROM. The enclosure works real well as it already had a switch, inputs and an output. The circuit is a simple single-ended circuit using a small mosfet as the output device. Most importantly, the amp sounds very good!

The site has very detailed instructions for making your own replica of this cleverly-encased amp. Not a bad choice of headphones, either.