I guess since I spend a lot of time driving and on the phone, my favorite 3rd party app would have to be DriveSafe.ly. It comes in handy and keeps my hands on the steering wheel
I conidered Quicklaunch too, but came to the same conclusion that SCrid2000 did. My primary use of QL is to launch 3rd party apps, without those, the built in hotkeys work for what the phone itself can do.
BergerKing brings up a good point too, and if I used my phone in that way QL would be my go to app.
Definitely Blackberry Traffic. I use it every time I drive to a new destination and it ALWAYS stop on. Went to go play a game of footie in a nearby city I've never been to and thanks to the app I didn't get lost.
Google Maps - the perfect app for road warriors. It is great for avoiding traffic delays in metro areas.
VLingo - for dictating text and emails while driving