Most cars these days come with a decent stereo, with at least a cd player and a radio; but not everyone drives a recent car. My current vehicle happens to be a pickup truck over 10 years old, so it only has a radio. I was disappointed when I first discovered that there was no input jack on the stereo, and I couldn't afford to put in a new one. Fortunately, I found a way around the problem that allows you to play cd's, audio cassettes, and even your iPod over your radio, without having to buy any expensive equipment.
The secret is a little device that transmits sound from an audio device -- cd player, iPod, etc. -- to your radio. It's like having a radio station in your car. You simply plug it into the headphone jack of your audio player, turn it on, and tune your radio to the appropriate channel. The most popular of these devices is probably the iRiver, but there are plenty of competing brands. I have an iRock, similar to the one seen here (mine looks less flashy, but allows you to broadcast on more frequencies). Both of them allow you to broadcast your music on various FM frequencies, so if you run into interference from local radio stations, you can simply switch to alternate, less crowded frequency. The iRiver is better -- it allows you to transmit on any FM frequency, and, from the reviews, it probably gives better sound quality -- except that it can't operate off of batteries.
It's cool that this little unit, which is smaller than a mobile phone, can send a signal to your antenna. What's even better is that it doesn't depend on any input from your stereo (except having a radio, which almost all of them do. Even if you have a cd player in your car, you may not have a tape player, or an mp3 player; and even if you have an mp3 player, you may not be able to play music directly from your iPod. This device solves that problem, and any future input methods that may arise. As long as you have a car radio, you can play your music.
This is particularly valuable for me, because I like to listen to audio books; and even though most public libraries have a decent collection, most of them are still on tape. (Libraries do not automatically update their collections to the latest media.) I've learned another trick that enormously expands the possible books that I can listen to. I'll share that discovery tomorrow.