I use the whole "smart tools" collection routinely in the field. Especially the smart ruler, compass, protractor, and bubble level. I also use "Steel Shapes" frequently to confirm beam sizes in the field. I second the New Millenium apps for joists.
Beyond that, I use the flashlight and calculator apps routinely but have used several and have no preference.
PE, SE
Eastern United States
"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi