I have a few useful ones that I have used. I like GasBuddy for finding cheap or any fuel depending on the area. I have ConvertPad for converting from US to Metric and vice versa. Fuelly tracks my fuel mileage. I use Waze for GPS as it is social and people can report traffic jams or speed traps. One I have not used but probably will when I buy a OBD2 adapter is Torque which allows one to access more vehicle data in modern cars.

1957 Chevy 3800 Panel Truck - "Nightmare"
(Nightmare to restore)
Will give nightmares to the Fathers of my girlfriends.
"I'm in artillery!" Thank you, Bob. Can we play anything for you? "Anything! Just play it loud, OK?!"