Very good idea for a thread, Richard!

I have a few must haves in my truck:

(Some of this is EMT/First responder, so might not be normal)

Multi-tool / knife
Flashlight
Extra sweater or blanket
First aid kit
Non-rebreather mask
BP cuff and stethoscope
Sam Splints - soft splints that are multi-purpose
Extra triangle bandages with safety pins
Sunscreen
Tool kit

If I plan on making a long trip, or if I'm going someplace I don't know I'll add:

A bag of food with beef jerky, hard candy, and some MRE style meals.
At least a gallon of water
Extra engine oil and radiator coolant
Rope
Electrical and duct tape
Exta fuses for the specific vehicle
Blanket

There have been some really good items listed in this thread. Another great thing to remember that if for some reason you do end up lost pull of your rear view mirror. It makes an excellent signaling device if the sun is out. Another trick that my grandfather taught me is to use the exhaust manifold to heat food and water.