for off road: (I have a long bed truck so it all fits)
power invertor w/ a drill and a clamp light is in my tool box. you never know....
Spare battery, alt, starter, axle shafts, fuel pump
Vice grips and tin snips
hacksaw w/ blades
bailing wire/coat hangers (great for ripped off exhaust mounts)
duct tape
random hardware in 3/8's 7/16, and 1/2 sizes with washers and nuts
Zip ties and wire connectors and alligator clips
tub of grease, rtv
5 gallons of gas in a kerosene container so no one steals it and its easy to pour.
freeze plugs
headlight and taillight w/ lens
ratchet straps
winch (mulitmount) with all supplies (straps/shovel/snatch block/chain/hilift/d rings/hitchpin/remote)
spare fluids including water
spare long spark plug wire
toilet paper (the ladies will thank you later)
cigarette lighter
hand soap
big zip lock bags for adding fluid in diffs and random bolts and stuff
tool kit with necessities including special sockets and picks and skrewdrivers
camera
fire extinguisher or 2
full size spare or really close in a 16 or 16.5 inch rim so everyone can use it in your group (if your all 8lug)
dd:
flashlight
duct tape
camera
spare tire
jumper cables
cheap tool kit for the car
lug wrench
credit card
cell phone