When I am shooting, I never leave home without my trusty Leatherman tool.
My boss keeps reminding me whenever I am packing his photo bag, make sure the torchlight (Maglite) is there.
My shooting buddy's (who is also my photo assistant) favourite MUST HAVE "tool" is a tennis ball-size Kushball attached one of the zippers on his photo bag. It is his lucky charm.
What's yours? I am sure this can be interesting.