If you have an iPod, and use it on long trips or will be away from a computer for a while, this External Li-ion battery would probably be very helpful to you. You can plug your iPod into the battery to gain approximately 6-8 hrs of playtime for your iPod, although with the flash-based iPods (nano and shuffle) you may get more extra time than that.
This battery is compatible with the: iPod nano, iPod 5G, iPod Photo, iPod with color display, iPod 4G, iPod 3G, iPod Mini, and iPod shuffle. The iPod shuffle can be plugged directly into the battery, however for all other models you will need the USB sync cable that you use to connect your iPod to your computer. The battery is charged the same way you charge your ipod through the USB port and has an LED battery level indicator.
It outputs 5V of DC power at a max current of 1000mA. The battery weighs just 11.03oz and could fit right in your pocket with your iPod. Also included is a USB-A to USB-mini-B cable.
I also think you could probably rig something up with this battery to do more than just power an iPod. I get the feeling that it is not completely intended for iPod use because of the USB-A to USB-mini-B cable which makes me think they have an open mind about compatibility.
All in all I think it would be an essential gadget for your iPod if you find yourself not getting back to a computer regularly for a charge. The Macally IP-A481 External Li-ion Battery is available from NewEgg for $34.99.