There seems to be a scattering of apps mentioned that members here use in their vehicles. Just going to start a thread to put them all in one spot for people to check out and give feedback on. Also note what phone you are using the apps on, specifically Android or Apple based.
Verizon Wireless Droid 1
#1 Pandora Radio
Free music that you program the stations for. It learns your likes and dislikes in music. Awesome app. Unlimited skips and no commercials for $36 a year. As soon as my Tundra gets here I will upgrade to the pay level Pandora One. ETA: You can use this app on your laptop or desktop too.
Pandora Radio - Listen to Free Internet Radio, Find New Music
Used for tracking gas mileage and operating cost of fuel
Mileage ~ Android Application v3.0.8 By Evan Charlton | Finance
Just like it says, free GPS navigation that is self updating. Uses Google maps and has yet to steer me wrong.
Google Maps Navigation for mobile
#4 Weather Bug
GPS based weather app that gives you current conditions, weather alerts, and forecasts.
Weather for Google Android Smartphones - WeatherBug
Yes, I am a cop, and yes I use this when working and when off duty. Yes it used for drivers to tell other drivers where radar is being run on the roads. However, it has many other notifications as well. I use it to show wrecks, road hazards, school zones, ect. It is like gas buddy as it depends on user input, but a cool app none the less. Available for both Android and Apple devices.
Speed Trap Sharing System - Trapster
I have more, but I'll give others a chance to share some input.