I hate radio. When you commute enough in one day, you realize that the radio plays a short list of the same music over and over again throughout the day. One alternative is to listen to CDs, but that means going through the hassle of buying blanks, making playlists, burning them, loading them in your CD changer… No thanks. Most people would much rather listen to their iPods in the car.
Unfortunately, even today many new cars don't come standard with a way to connect an iPod. The ideal setup is over bluetooth for wireless listening. Next is over a dock connector that charges your iPod while in the car. Another option is through a standard mini-stereo cable that plugs into the headphone jack. A lot of people can't do any of these.
This is where an FM transmitter comes in. They all plug into the cigarette lighter and act much like the standard iPod dock. Once connected, they broadcast whatever is playing over an FM frequency that you can tune to on your car radio. The quality isn't as good as over a cable, and you'll sometimes get interference, but it's worth it. Instead of awful radio, you get to listen to your entire music and podcast collection. And even better, if you have an iPhone, streaming music apps like Pandora work great over 3G.
The best FM transmitter I've found is the Belkin TuneBase. It's by no means perfect. I wish the design were sleeker and the plastic construction is a little flimsy. But it works, and it's reasonably cheap.
Belkin TuneBase FM Transmitter: $65 – Buy Now