Griffin Technology's PowerJolt car charger
Already popular among the iPod crowd, Griffin's PowerJolt car charger has been customized for the iPhone.
Just plug the PowerJolt into your car's cigarette lighter or power socket and you can charge your iPhone even while you're using it. And the in-line 2 Amp automotive fuse means that you don't have to worry about power spikes or surges.
There's also a detachable dock connector cable that can be used separately for charging and syncing to your computer, meaning you pretty much never have to worry about a low battery - and isn't that the point?