Podshanking

Transferring music files directly from one iPod to another. First-generation Podshanking uses a cable from the headphones jack of the source iPod to a voice recorder accessory on the destination iPod.

Not the Best Quality
The digital to analog conversion to the headphones combined with the low-rate digital recording yields significantly lower sound quality, and the transfer is real time: you have to play the entire song. Using iPodLinux and other hacks, effective digital file transfer is expected (see iPodLinux).