The new app I downloaded for this thing was Road Ninja. I was searching for an app like this a few months ago and ended up with something that left a lot to be desired. I can tell immediately that Road Ninja is superior -- to start with, it's pretty! Visual appeal and intuitiveness is huge when it comes to mobile apps. It's hard to forgive an app for being ugly and difficult to use. Road Ninja appears to be accurate and well-organized, and I can't wait to use it on my next roadtrip!
Spotify is an app that I've already been using ever since it became available to U.S. citizens (it was available in Europe for several years prior, it seems). I use the free version only, which means that I can't choose a specific song and play it immediately, but I have created a starred list of the favorite tunes I like to sing along to and that is great for listening in the car when I'm not listening to a podcast of some kind.
I would like to recommend another hobby app, Geocaching. It is a paid app, unfortunately, but absolutely essential for finding and logging geocaches on the go, especially if you didn't take the time to load up your dedicated GPSr unit or are stopping to find a cache spontaneously while out and about.