My common sense. Having the ability to think through a problem and come up with a solution.
cell phone, that's my office. as for out in the field, caulk gun, I can improvise for just about any other tool, except my caulk gun.
Tape measure and cell phone...Though at times I would love to heave the phone against the wall.
Pen,paper,and reading glasses. The reading glasses I can do without if it is a bright sunny day. Pen and notebook and glasses are always with me.
... ladder...
Litterally. I'm certain that without it, I'd be dead by now.