If you're not a fan of DIY kits like the Mintyboost to give your portable devices more juice on the road, this external battery from Macally may be more to your taste.
The product page states that it's "for" iPods, but the miniUSB port and the included cable should allow many other USB compatible devices to be charged. On a full charge, this battery adds 6-8 hours of play-time to your hard drive based iPods. There's also a LED battery indicator so you know how much juice is left.
Useful device for mp3 players and cellphones that can be charged via their USB ports.