I'm not a fan of FM transmitters, because they never seem to work for me. Maybe it's because I live in Los Angeles, and the limits placed on them by the FCC render them less useful in a crowded market like this one.
But after trying them all, I've found the best. YMMV, but I love the DLO TransPod. It broadcasts clean and clear, and has an input jack for a non-iPod device. It comes in black, silver and white.
Another useful tool is the Sirius Frequency Finder website. You can punch in your zip code and get a list of the best frequencies for your FM transmitter. I was able to find a frequency that was much clearer than the one I had been using. (I feel a little guilty: I'm an XM listener.)