Here is a list of things I was thankful to have during my off roading disasters:
Water - like five gallons When working hard to get unstuck, water is nice. Especially if its 110 degrees which is usually how hot it is when I get stuck (that or 15 degrees).
Snack Food - also good for energy when working hard to get unstuck
Extra set of boots/gloves/jacket/hat/blanket - staying warm and dry is good for survival when your truck is stuck in a snow bank
Spare Tire - same size as the other four, plus you can use it for all kinds of non-tire things, like a high lift stand in the mud/snow or a winch anchor
As much recovery equipment as possible - tow straps, AT LEAST two chains, D shackles, tree strap
Hi-lift - Jacked and winched me out of a bind too many times. Plus the handle makes a good breaker bar.
Shovel - number two recovery tool next to a hilift
Bowsaw - Safer/easier/faster than an axe
Tool Bag - enought tools to dissasemble ANYTHING on the vehicle - suspension, steering, axle, transmission, whatever. For mine, I only need a small toolbag to hold it all
Heavy Wire
Random hoses/fuel line for repairs, siphing and stuff
Small Electrical Kit - fuses, spare wires, connectors, strippers
Fluids - Gas, Oil, Gear Oil, Brake Fluid, Grease
Gasket maker
First Aid Kit
A friend (who's smarter than you) preferably one with his own vehicle.
Stuff I didn't have but probably should've:
A maintenance checklist with everything checked off before leaving
a cell phone - though I don't have one and usually don't go places that have service
a 12v air compressor and tire repair kit
a laminated map on the wall at my house with the area where I was going circled with a dry erase marker (my family wouln't have a clue where I went if I just told them)
All of this stuff minus the shovel, hi-lift, and spare tire fit easily into a truck tool box.