Many car owners take their built-in auxiliary inputs for granted, but there are still plenty of vehicles that don't have them, leaving the job to FM transmitters. If you're vehicle is in that group, then playing back audio from a smart phone or MP3 player should be easy to set up with any of these.
Belkin TuneCast Auto with ClearScan for iPhone and iPod
Available at: Future Shop, Best Buy, The Source and Wal-Mart
This transmitter works with a free GPS-assisted iPhone/iPod touch application that finds the clearest FM signal depending on where you happen to be located. This combination works well but the key seems to be placement of your iPhone/iPod in the car. For example, leaving it on an empty passenger seat seems to cause more interference than resting it in your lap.
The good news is that any audio (except calls) streams through your car's system. This can include Internet radio and sound from videos. And since the TuneCast also charges the iPhone/iPod's battery, playback isn't impeded by a lack of power.
Although there is no way to use the ClearScan app with an iPod Nano, Classic or Shuffle, finding a good signal and sticking with it isn't difficult. The longer cord might also cause annoying entanglements in the car, but it's usually pretty manageable.
Griffin iTrip Auto Universal Plus FM Transmitter and Car Charger
Available at: Best Buy, GriffinTechnology.com
So long as your phone, MP3 player or mobile device has a 3.5-mm headphone jack, you will have no problem playing back audio using the iTrip Auto Plus. There is interference depending on where your device is placed, but it's easily remedied by just moving it around. The iTrip will automatically scan for the best open frequencies through the SmartScan button, which it tends to get right most of the time.
An added bonus is the USB port built into the plug, which makes it possible to charge your device at the same time. This works seamlessly, except that having an extra cable can create a bit of a cluttered mess.
The iTrip tested here was the 2009 model, which is not to be confused with an earlier Griffin product that bears the same name, yet isn't as good in performance.
Scosche Digital FM Transmitter with 12V Flex-Mount Neck
Available at: The Source
Like Griffin's iTrip, Scosche's transmitter has a regular line-in cable that plugs into any 3.5-mm jack, meaning it will work with any MP3 player, phone or portable device that has a headphone jack. For whatever reason, you have to really raise the volume on your car stereo to get your mobile device's audio at a good audible level, though distortion seeps in sometimes.
The flex-mount neck makes it easy to plug this in the way you want, and the LCD screen looks nice. But unlike the other two transmitters, this one has no ability to charge devices connected to it, so playback could be limited unless you have another way to simultaneously charge your mobile device.
It doesn't auto scan for open FM frequencies, and limits you to three presets, but as a budget buy, this transmitter is still a capable unit.