I use a AA battery pack I bought at
www.gomadic.com.
It uses interchangeable tips, so you can use it for pretty much all of your gadgets. I have tips for my iPod, Archos 605, cell phone, Kindle, Sony Reader, and some generic USB ones.
The battery life is great if you use lithium, and it will use rechargeables too. There are some new nimh rechargeables I got off
www.thomas-distributing.com that are low discharge, which means they hold their charge for a year! No more dead rechargeables after a few weeks.