I EDC (every day carry) a Victornox tinker Swiss army knife in my pocket.
...in the vehicles;
tools sufficient to remove and replace nearly every fastener (wrenches sockets screwdrivers etc.)(fits in a medium ammo box).
an old D cell G.I. angle head flashlight converted to LED operation (water resistant and too ugly to steal, it can shine up, at an angle or down and is mostly hands free).
a DVM with vehicle schematic,
tire plug kit with extra plugs,
tire irons,
a portable air compressor (or a converted AC compressor under the hood),
A U.S.G.I. matt-ax or G.I. mini pick
a G.I. trifold entrenching tool
A Svensaw (large)
A good set of heavy duty jumper cables,
if four wheeling; a stinger a, welding rods and welding goggles (#10 lens), use with jumper cables and 2 starting batteries.
the winch accessories & other recovery gear, tree strap, snatch strap, snatch block, etc.
a log chain, good for rigging the snatch block for the winch, or with a bolt to tension it can clamp.
Spare universal joints for drive shafts and axles.
spare u bolts or clamps for the joints.
spare fluids, a small quantity of axle grease and radiator stop leak.
a set of hoses (get universal flex hoses for the big radiator hoses, they store better) with high quality hose clamps.
epoxy putty
A length of high pressure fuel line capable of reaching from the tank to the engine with a spare high pressure injection pump.
A peanut butter jar full of misc. nuts bolts washers spar spark plug, tire valves & stems, electrical crimp on splices connectors etc, and random bench top junk.
When four wheeling depending on the trails; a spare drive shaft.
Flat items are often Velcroed (Velcro one-wrap) to the roll bar, the tool can is bolted to a fender, the hoses are secured using their hose clamps to struts under the hood. The fluids ride in a milk crate.
this doesn't count commonsense items like a fire extinguisher and first aid kit (or toilet paper)...
Enjoy!