TomTom USA & Canada
At $59.99 plus a $20-a-year subscription if you want real-time traffic updates, this is the most expensive app in this list, but it may also be the most powerful. TomTom USA & Canada uses the GPS in your iPhone to turn it into, well, a TomTom GPS. So if the app seems a bit pricey, consider how much more it would cost to buy the GPS device itself.
Once installed, TomTom USA & Canada does everything a good automobile GPS should do. It calculates routes for you (and lets you customize them), depicts the routes visually on your iPhone's screen as both 2D maps and 3D lane displays for the freeway, and shows useful information like when you'll arrive at your destination and how fast you're traveling. You can use the iPhone's touch-screen to set destinations and you can listen to clear voice instructions (which in many cars can be output through the radio or Bluetooth interface) that will briefly fade any music you might be playing so that you won't have any trouble hearing them. You can even get voice instructions, if desired, while making calls on the iPhone.
Be warned that TomTom USA & Canada is a memory-hungry program and will immediately zap 1.5 gigabytes of your iPhone's memory for the app itself plus additional space for data. So the extra cost of turning your phone into a GPS may be saying goodbye to a large chunk of your favorite music and to that copy of Angry Birds you were planning to play at the next roadstop.
Once you're off on that trip, you might start worrying that you left the garage door open. Wouldn't it be nice if you could check without calling a neighbor or driving all the way back home? Now you can -- with the next app.