My iTrip iPod FM Transmitter and Car Charger allows me to both listen to my grooves and charge my iPod while driving. It finds an available FM frequency, then transmits your music from the iPod and through your car radio. Why, you might ask, is this green? Because I am no longer burning CDs to play in the car!
I have had few problems with sound clarity and it's easy to use. Also, if you are someone who actually shuts down the computer and unplugs it to save energy (rather than leaving it in a constant state of hibernation, which draws power) then powering back up just to charge your iPod is annoying. The iTrip solves two problems in one fell swoop. I recommend it!