Premium Rapid Rapid Automobile Car Charger FM Radio Adapter Transmitter For Apple iPod / iPhone / iTouch / Nano
This is the second one of these I have purchased, simply because the price is right.  Sometimes they work well, but it really depends on the radio frequency you choose, your location (you might have to change frequencies as you drive), and the position and orientation of your ipod.  Someitmes it works well in the passenger seat or center console, and you have to rotate your ipod around, but often there is static (sometimes a LOT!).  Another complaint is that if you use a case for your ipod (and who doesn't?