back in the days of assisting, i had my pager, my thomas guide and rolls of quarters to use in pay phones...thankfully those are days gone by. today, life is so much easier and with these apps on my iphone/ipad it is almost too easy!
sunrise sunset: takes the guess work out of how early i need to wake up for those chilly morning shoots and those magic hour sunset images.
clinometer - great for leveling what ever it is you need leveled, like a camera.
google earth - amazing...find trails, terrain, etc...of course you need a cell signal.
ifotoguide - now i've been to yosemite and arches without these apps and fared well, but really look forward to going back to both locations with these on my iphone. in addition to popular photography locations, ideal shooting times and directions, it has additional resource links.
walmart - just because it is so wrong! seriously, no matter how much i pre-plan/pack, i always end up forgetting something and undoubtably, there is a walmart nearby that saves the day that just happens to be open all hours of the night.