One of the big pains in the ass with having an FM transmitter to get music from your iPod/Zune to your car radio is finding a clear radio station. A frequency may be fine where you live, but drive 20 miles in any direction and it could be totally packed. DLO's OpenFM tool lets you search by city or zip code and gives you the best (most open) frequencies in the area to tune your FM transmitter to. And they're hoping that you buy a DLO FM transmitter while you're there. It even has an iPhone-customzied version. [DLO]
AU: Thankfully our more centralised radio networks mean we don't have such drama. But it's still a pretty cool tool, though US only right now.