[Jesse] sent in this headphone amp. It's really just a board with a dedicated smd headphone amp chip(MAX9725) and a pair of smd caps recycled from an old hard drive, but it does job. I think the goal is to boost low signals rather than the usual audiophile quest for cleaner tunes.
http://www.hackaday.com/2007/08/20/tiny-inline-headphone-amp/