Basic Equipment
Every vehicle should carry these and they should be securely fastened down (like everything else).  You should be able to reach the flashlight and fire extinguisher from your seat.  Factory seat belts do not release if you are upside down (don't ask how we know this).

• CB radio
• Flashlight
• First aid kit (include any of your family's special needs like EpiPens or insulin)
• Fire extinguisher
• Complete change of clothes
• Full size spare tire
• Jack
• Wrench to remove tire (and key if you have locking lug nuts)
• Tow hooks front and rear
• Specialty tools for your vehicle

Good to have
With the exception of the Hi-Lift, every vehicle really should have these too.

• Tow strap (no hooks on ends!)
• Shackle
• Short piece of chain with washers and bolt that fits through middle links
• Small tarp or blanket
• Hand towel(s)
• Rachet strap
• High lift jack

Basic Tools
Regardless of your mechanical skills you should carry these to fix your rig of someone else's. 

• Philips and flat screwdrivers
• Adjustible wrench
• Set of combination wrenches (metric or SAE to fit your vehicle)
• Pliers
• Ball peen hammer
• Duct tape
• Zip ties
• Small roll heavy wire
• Spare fuses
• Fan belt(s)

Optional Tools
These are really more for the skilled mechanic, but consider carrying a few of these as well.

• Schrader valve and removal tool
• Chisel
• Punch
• Socket set
• Small vice grip
• Channel locks
• Allen wrenchs
• Torx bits (if your vehicle is cursed with them)
• Stubby and offset screwdrivers
• Hacksaw
• File
• Teflon tape
• WD-40 or wire dry spray
• Scissors
• Electrical tester
• Wire strippers
• Electrical wire, tape and  connectors
• Misc. nuts and bolts
• Extra fluids (motor oil, brake fluid, power steering fluid, ATF, gear lube)