Got a cassette player in your car, but all your music is digital?  Have you found FM transmitters to be a less than perfect solution to getting your music on the car stereo?  There's another option: The ION Bluetooth Cassette Adapter.  http://www.ionaudio.com/products/details/cassette-adapter-bluetooth

It looks like a normal cassette tape, but is actually a Bluetooth receiver.  Just charge up the built-in battery, pair the cassette with your smart phone or mp3 player, and you're ready to go!  Once you've got the cassette playing in your car stereo, any audio output from your phone or mp3 player will be heard over the speakers.

Unlike FM transmitters, you aren't reliant on the cigarette lighter for power (except maybe to charge your phone/mp3 player).  You also don't have to compete with other FM transmissions on or near your chosen/designated station.  I don't know about others, but my FM transmitter will often "pop" or "crackle" from interference, and sometimes get completely overpowered by other broadcasts.  Because of the short-range nature of Bluetooth, this problem is avoided by using the cassette adapter.

The cassette adapter also includes a built-in mic which you can pop out and use as a hands-free interface for your cell phone.  I can't speak to the quality of it, since I haven't tested it.

The Good: Nice and clear audio from my mp3 player, wire free and without FM competition
The Bad: Battery supposedly lasts 4 hrs.  Would've been nice to have a dynamo charger attached to the cassette wheels.
The Ugly: You have to supply your own cassette case.