I just got back from a 6-week cross-country road trip with my boyfriend, and we both had our shiny new iPhones to assist us on the way. Here's a list of some of the most useful apps that we'd recommend to anyone road tripping with an iPhone:
1. Pandora -- so much better than the radio, and it often manages to keep working even when your data signal and/or phone signal are weak-to-nonexistent.
2. Mint.com -- a great way to stay on top of personal finances while on the road, with the ability to check all your accounts in one place (*does require some set-up before you go if you don't already use Mint).
3. The Weather Channel -- as long as you have a strong data (3G or strong Edge) signal, a great way to track storms where you are and where you're going.
4. Flashlight -- when camping, the flashlight app was almost as strong as our heavy-duty flashlight, especially on a new iPhone with the LED flash.
5. Yelp -- a helpful tool for finding attractions, hotels, and restaurants with reviews, addresses and maps.
6. Kayak -- for the most part, a good resource for finding hotels (once in a while it wasn't accurate re: availability, but generally it pointed us in the right direction).
7. Priceline -- often unhelpful, but it snagged us a room at a Hilton in Omaha for super cheap, so that one success made it worth our while.
8. OpenTable -- especially after some kinks got worked out on the newest version, a great way to make reservations on the fly and rack up dining points (we earned a pretty nice gift certificate just by using it during our trip).
9. Maps (standard on the iPhones) -- much easier to use than MapQuest; just be sure to map your routes or focus in on your location each time you get 3G so that it will retain the map info even if you move into a bad/no service area.