I use the iMap weather radio app that sends notifications (text and audible) of nasty weather such as tornadoes, snow, ice, and can be customized for any sorts of conditions. It pulled me off the highway when I was in the path of a twister in April. Costs $10 but priceless in my estimation. I was heading out this morning when an alert came up for freezing rain. My regular weather app said nothing about it. It's nasty out there. Glad I wasn't out in it.
AAA roadside assistance app. Connects you to assistance. Informs them of your location with GPS. When I lost my brakes on the way to Texas in the middle of the night I didn't know where I was to tell dispatch. The truck found me in minutes. App was free.
My Pilot app gives you the locations of the pilots and flying J's on my route. Tells you what their gas prices are too.
GPS plus (Telenav) cost $10. Love the spoken directions, "search your route" function, many icon choices (mine is a Star Wars type fighter jet that cruises just above the interstate), and the choice between a female voice and a British male voice. The girl gives turn by turn directions, the boy simply states your location but doesn't give directions
Walmart app has a locator that I have found to be incorrect at times. Took me to the middle of a cornfield at 2 am in Waco Texas. Miles from the actual super center.
My new favorite is the Walgreens app. I switched my prescriptions to them because of this app, and the customer service. I have full access to all prescription info via the phone. Renewals are by tapping screen icons or by scanning your prescription bottles, and can be transferred anywhere in the country with the push of a button. It can be done ahead of time, so I can refill my scripts in a truck stop on Dallas Texas on Friday and pick them up in San Antonio when I arrive the next day. My rewards card can be scanned from my phone, and all current coupons available on my phone also. If I want photo prints, I send them ahead of time and the prints are waiting when I get there.
Netflix. I am grandfathered into Verizon's unlimited data plan and can watch movies all day. Subscription to Netflix $6.99.
Stores that I am likely to shop at (target, Joann's, etc) have apps with coupons so I don't miss out on the mailed coupons I used to get.
I have a folder of business apps that make doing work on the road easier. Apps for eBay, etsy, PayPal, USPS, fee and seller calculators make it so simple. I have a new app called PayPal Here. They sent me a small scanner the size of a quarter that plugs into the earphone jack. I can take debit and credit cards with that and have instant access to the money with my Paypal debit card. I can also take checks anywhere on the road and they are scanned in and processed with no need for a bank. I adore this app. I can use it on the side of the road, at flea markets or craft fairs.
Those are my main ones, mostly travel related. I use the kindle app for storing books and documents and patterns.