I saw an interesting gadget at the local Walgreens. They sold the cord for a phone charger, with the phone plug on one end and the USB jack on the other. In the next bin, they had just the part that plugs into a car charger for $5. It had a USB port, so you could plug in any phone that charges via USB.
This looked like a great idea to me because I can use my iPhone's factory charging cord in the car and don't have to drop $30 for a name brand charger, or even $15 for a generic. Plus, one wouldn't have to buy two car chargers, or switch from one car to the other. At $5 a pop, just buy one for every car and carry the cord from car to car. No big deal.