Bacpac.

Are you looking for a portable battery charger that will then be used to charge your MP3 player? A friend carries around a Motorola power pack and she has had mixed results with it as these power packs need to be periodically charged also otherwise you end up with 2 batteries that are low on power.

Another decent brand portable power pack that wont break the bank is the Enegergizer XP2000.

For most people though, having a 120V, 12 volt car charger or a 12 volt to USB charger will suffice as it will not be too often that one would find themselves away from any of these power sources.

In my case, at home, I use the 120V charger that came with my cell phone. For work, I have a spare usb cable that I use to plug my phone into my desk or laptop computer. For the vehicle, I use this 12V to USB adapter which can also be used for any other USB devices.