From Amazon.com. I have purchased these two and have tested them., They work.
Kensington Car Charger for Mobile Devices with USB Port - 9.99 free ship
This charger has a standard USB port and fits a car power outlet perfectly. In fact I am going to get another and leave them in both cars.
Tekkeon TekCharge MP1800 - 36.90 with free ship (thanks to willanaya for bringing this product to my attention).
This "extra battery" is actually a stand-alone battery that could charge the Droid multiple times. It comes with various charge connectors the most important being the mini USB and micro USB (for the Droid).
The Droid comes with a wall plug with a USB port. The MP1800 is charged from a mini USB port but since it comes with a USB cable and a mini USB connector, you can charge the MP1800 using the Droid wall charger. Once the MP1800 is charged, it can directly run the Droid or simply charge the Droid battery.
The MP1800 is a little bulky to carry in your pocket. It is also a flashlight. I think I need to get another wall charger with USB port so I can have one in my office and one at home. I am definitely getting another car charger (at 9.99) and another USB to micro USB cable and a USB to mini USB cable so both cars will have a complete charger for either the Droid or the MP1800.
I am planning a 9 hour driving trip and plan to use the Droid to listen to my Pandora stations. My car audio system has an aux input port (I purchased a cable from Radio Shack) so I can plug in the Droid. The car trip will take me out of any ATT coverage but not Verizon coverage - so I expect to be able to listen to Pandora the entire way.