I have been extensively using my phone while driving around. Here are a list of apps I found extremely useful:
1. Google Navigation - of course
2.
Dailyroads Voyager - To record the video and audio, runs in background. You can playback later along with speed and gps co-ordinates. Will retain the video automatically in case of a sudden braking event.
3.
Google MyTracks - Once your journey is logged, you can open it in Google Earth (either mobile or PC app), and retrace your driving like in the movies. The best part is that it shows a graph of your speed and altitude, and you can easily see where you were speeding the most. Note that it may be a bit difficult to locate the play button in Google Earth.
4.
iOnRoad Augmented Driving - It can warn you if you go near another vehicle, or depart from your lane. It can also remember your parking spot automatically. Its not 100% perfect, but better than nothing. There are two problems though - I cannot run both DailyRoads and iOnRoad, since both require camera access. Also, iOnRoad causes the phone to heat quickly.
Also, please check out
this link for a bunch of privacy related apps. I especially recommend Cryptonite. I am trying in right now, and it seems to give the same functionality as TrueCrypt in PC. IPTablesLog is also good - you can see what all apps are accessing internet.