Griffin USB 12v Adapter
This is a simple way to add 2 USB ports to your vehicle. All you need is an available 12v accessory outlet in your vehicle, or a cigarette lighter, and you're set. I use this charger on a daily basis for my iPhone 4s, and my iTouch. By providing 2 USB ports, this allows your passenger to also charge their device as your's charges.
I haven't noticed any charging issues with this USB adapter. The only problem I have noticed with this charger is that it can be difficult to remove once it's installed into the 12v accessory port – because there's nothing to grip on to in order to take it out once it's in. I have only noticed this problem with my 2005 Dodge Ram, it seems to slide in and out fine in my 2001 Jeep.
Overall, this is a simple solution to adding USB ports, and it only takes a second. You can't beat this for convenience.