I bought this FiiO E1 Headphone Amplifier from an internet guy this week. Cost me SGD29.
One end is plug into the dock of the iPod or iPhone. The other end, you plug in your earphone. So it sits between the iPhone and the earphone. It is supposed to improve the sound quality. Perhaps my ears are not as good but I did not really notice any difference. Perhaps it needs to run it also ? :)
Anyway, what I do like is because of the in-line control function, you can easily control the playback of the iPod/iPhone. The controls of my Apple in ear headphone has not been working for a while so I am so happy to find a cheaper alternative :). Enjoyed skipping tracks and controlling the volume the whole day :)