I would add...
In addition to chnlisle's list, I carry combination wrenches, 2 different size channel lock pliers, several pieces of rope in various sizes from 1/4 inch 10' to 5/8" 50', a set of links levelers (mostly to level the truck in camp, but can also be used to put the jack on in loose dirt), wire stripper/cutter, electrical tape, 12" wire ties, a couple of pair of leather work gloves, a 2" receiver mount clevis hook, a 4 ton come-along, JB weld epoxy, a 12V air compressor, and a few spare parts like my old serpentine belt, bulbs, fuses, engine oil, trans fluid, brake fluid.
I would rather have a winch than the come-along, but I've only used it once and find it hard to justify the cost. I also have a 2" receiver in the front of my truck.
My entrenching tool has a pick on it.
If I'm not going 4x4 camping I may leave some of the recovery stuff behind. However, once on a road trip I pulled out some people who had pulled to the shoulder in very soft dirt and were stuck before they even knew it. They were glad I had the strap and clevis hook.
My recovery strap is non-streching, I've been thinking of adding the stretching kind after I saw a guy pull 2 stuck jeeps out of the mud (the second one got stuck assisting the first) by hooking up to the second and getting a good running start. Of course the other 2 jeeps were ready to go as soon as tension was placed on the line, so it wasn't only the unstuck jeep that did it, but still, he had bragging rights no one could counter.
Most of the stuff fits under the back seat, but some of the stuff that's good for camping (entrenching tool, levelers, saw, etc.) goes into the camper.
That's all I can think of now. I may look in the truck tonight to make sure I didn't miss anything.