First of all.... I don't use all of these apps while driving, and I'm not telling you to do so. Here is a list of some of the Apps that I have installed on my iPad 2 and my girlfriends iPad that we use in the vehicle that work great!
Co-Pilot (Navigation system, that is constantly updated)
VEVO HD (Music videos that is totally free and updated daily)
Beat the traffic (traffic guide with live images of roads)
Netflix (Use this in the front for passenger of headrest)
Google Maps (works excellent to see what the building looks like before you get there)
2do (sync's notes, todo's etc with iMac, Macbook, iPhone)
Zillow (home prices. Good for a day where you are bored and wonder how much some of the houses are worth)
SiriusXM (don't really use much anymore because free subscription is over..)
Trapster (only really use on long road trips)
Yelp (great app for iPad to find good restaraunts with reviews)
I have many other Apps on the iPad's but these are the most used.