FM transmitters, step aside. Bluetooth audio transmitters are the wave of the future. Not too many of them out there, but Scosche has just released a Bluetooth wireless interface for iPod and other MP3 players that will enable streaming audio from your portable player to your car or home receiver. It connects directly to the headphone jack, and transmits to a receiver connected to either a car's head unit or home audio system.
There are two car kits available; one is made specifically for the iPod, and the other is for any MP3 player. They sell for $249.99 each. The home kit for the home receiver is $129.99 and $249.99 respectively. Yes, that's pricey, but consider the superior quality you'll get from them as opposed to that junky FM transmitter you bought 2 years ago.