Another vote for the Engerizer Xpal series. I have two: an XP18000 and an older and smaller one called a Harry (?!).

The Harry, aka XP2000, looks like this:



It's a 2000 mA lithium battery, about the size of two matchbooks, and perfect for powering a cellphone, smartphone or iPod. It charges via a mini-USB input and can charge from either a laptop or a plug-in 5v power supply. Its output is a standard USB jack.

The XP18000 looks like this:



It's an 18000 mA lithium battery, about the size of a PDA (though thicker and heavier) that can run a laptop. I charges from a 19v source, i.e. a laptop power supply. It has multiple outputs, one for powering a laptop, another for a 9-12 v device, and a USB jack.

Both come with with monitoring lights that indicate the power status, and nice little pouches for carrying. The XP18000 also includes a variety of cables and adapters for powering a range of different devices.

I use these when I fly transpac and they've never let me down.