The Boostaroo is a pocket-size portable amplifier for your earphones. It amplifies the sound coming out of your iPod or MP3 player by 100%. I’ve tested this device, and it certainly does amplify quite well, and two AA batteries it uses last for weeks.
It also functions as a splitter and allows you to share your music with two other people (3 mini-headphone jacks total).
It increases the battery life of your MP3 player, because the player does not have to function as the amplifier. It does not distort the frequency response of the audio (the specs are: 20 to 20,000Hz +/- 2 dB).
Home Page: Boostaroo.
Available from Amazon for $26.