OK I'll add some stuff here as well I suppose...
This general list is a good idea just to get the juices flowing but three major things need to be addressed. If on is out and breaks down you have to address (in advance) you decision tree:
1. Stay and Play (fix the Jeep on site): may not be possible... but may also be your only feasible option if you are alone, far from anything, and on a DIFFICULT trail. This is also the only option that would allow one to progress as though nothing had happened.
2. Limp Home: do whatever it takes to get the vehicle rolling. This can include towing, pushing, or dragging and damage is a likely result. If someone is sick or injured this may be your only option. Also if you are aware of your mechanical abilities option 1 may not exist (i.e. not necessarily the time to bust out the Haynes manual and learn what you're doing)
3. Ditch the rig: This can be as simple as catching a ride back to town to get the parts necessary for #'s 1 or 2. It can also be a big decision should this be a worst-case scenario. (solo trip with family... 100 miles from anywhere... transmission puked a big pile of metal parts straight through the pan)
Every time you go out you should already know what your options are here. These should be your key factors in deciding what to bring.
There are certain basics I wouldn't drive down the highway without though.
1)Recovery strap
2)basic tool set (including knife)
3)wire strippers and electrical tape
4)5,000-10,000 calories of any sort
5)Extra warm clothes (includes boots, gloves, hats, etc)
6)Cell phone
7)Flash light w/extra batteries
Jumper cables
9)1st aid kit w/ bug spray and sunscreen
10)Fire extinguisher
11)spare fluids (specifics up to you)
12)Ratchet straps, bailing wire, parachute cord.... something like that
13 )Sleeping bag
14)Tarp
15)Big red atlas of whatever state you are in
16)compass
17)Road Flares
18)Duct Tape
19)Rain coat or poncho
20)roll of blue shop towels
21)Shovel
22)waterproof matches
23)water...amount up to you
Any additional items you bring should revolve around your situational decision about the feasibility of those 3 options listed above... for the sake of brevity I won't go into it all