If you want to listen to your iPod or iPhone in your car, you have precious few options. You can spring for a pricey stereo dock that requires pricey installation. You can use one of those tape-deck adapters, assuming your car is old enough to have a tape deck. Or, you can try an FM transmitter, which broadcasts audio from your player to an unused band on your stereo. Buy.com has such a product, the aptly named Av Lab iPod/iPhone FM Transmitter, for free after a mail-in rebate [PDF].
Compatible with most iPod and iPhone models, the transmitter features a backlit LCD and plus/minus buttons for setting its broadcast frequency (anywhere from 87.9 to 107.9). Find an unused frequency on your stereo, set the transmitter accordingly, and presto: your music should come piping through your speakers.
Full disclosure: In my experience, FM transmitters generally suck. But your mileage may vary. And, hey, it's a freebie! After you pay your $14.99 (shipping is free), you get back a check for $15. So you actually pocket a penny. Heck, it's like you're getting paid to buy this! (I know, I know, the stamp...)