Duct tape.
Wire.
Various sizes of hose clamps.
(I hesitate to say 'bailing wire' since there is much better grades of wire out there, but you need the wire to hold together the stuff Duct Tape won't)
I've limped back to town on a Duct Tape fan belt.
One of my big gripes is people who do not even carry the basic tools:
An adjustable wrench
A pair of decent priers
Screwdriver (the cheap-ass kind with mulitple bits can be nice)
Knife
Some kind of tape
Replacement fuses for the car they are driving
A flashlight that works
Amongst the basic tools I carry is a cheap POS breaker bar specifically to get the lugnuts off.
Sure most of you have taken off your wheels many times, but there are probably some that haven't. If you've had your tires rotated or replaced by a shop and you haven't retorqued them yourself then you could be in for a real surprise trying to do it on the trail (or the road) with the wrench the factoy provides.
If you don't have at least one working flashlight in the truck, you are screwed!
A Jack.
Make that a jack that actually works and works for your truck.
A spare tire that fits the truck you are driving
The spare must have enough air in it to support the truck.
A cell phone.
Sorry these are so basic, but for each of these items I've listed, I have a story regarding somebody who was immobilized by the lack of of one or more of these.
Roll of paper towels.
Toilet paper (no expanation required)