MacNN reviewed the Kensington PowerBolt Micro Car Charger. The charger sits flush on the car power port, with a female USB port capable of powering any mobile device that connects via USB cables. The charger comes with a detachable USB-to-iPod cable that powers iPhones and iPads -- the latter device getting a fast charge from the 2.1 Amp PowerBolt.
The PowerBolt installs easily, but removal is challenging due to its low-profile design. It sells for $25 and is shipping now.