TomTom USA: The iPhone comes with built-in GPS, and there's no better way to use it than with turn-by-turn directions. TomTom USA gives you up-to-date maps, and adjusts to any changes in the route you may take. The next time you go over the river and through the woods to Grandma's house, make sure this is running on your iPhone so you don't take a wrong turn.
SiriusXM Internet Radio: Admittedly, this app is a subscription service, but for just a few bucks a month you can get live sports, amazing talk and commercial-free music delivered to your iPhone on the go. Now you don't have to worry about hunting around for a radio signal wherever you are, you just have to have a decent cellular connection. I personally use it every day in my truck, streaming the signal to my car's stereo via Bluetooth.
Find My Friends: Getting to your destination is just half the battle. Once you're there, if you have relatives with iPhones you can sync up using Find My Friends. Just add each other to the service (which you can do temporarily if necessary), and the next time you head off to the mall or major event you won't have to worry about getting separated and lost. Plus, it's really cool to show to older relatives at the dinner table, and point out where people are back home.
Foursquare: You're in a different state, why not keep up on your checkins? For the uninitiated, Foursquare is a social check-in service that gives you points each time you check into a location, be it a restaurant or just your cousin Bruce's house. You get extra points for being the first among your friends to check into a location, and can even get discounts from certain businesses in the process.