The Hitch iPod2iPod USB Transfer Device is an interesting little device for copying songs between two iPods without a computer in between. You just plug both iPods into the Hitch and you can "select and transfer songs, albums and videos between the two iPods" directly. The Hitch has a little LCD for selecting songs, and even runs a version of Linux, presumably opening up many cool hacking possibilities. In fact, the Hitch works with any two USB Mass Storage devices, like USB external hard drives or USB thumbdrives.
Now for the bad news: any FairPlay music will not play until after a sync (though unprotected files will play immediately), and worst of all "Hitch will only work between two iPods that have been initially configured under Windows OS," which is to say only on iPods that are formatted with FAT32 and not OS X's default HFS+.