1. AAA premium membership w/ 100 mile free towing
2. Cell Phone
3. OEM tool kit & jack
I also always carry a basic tool kit w/ wrenches, sockets, etc., volt-ohm meter, some fluids (gallon water/coolant, two quarts oil, power steering fluid (ATF), brake fluid), new fan belts, fuel filter, representative mixture of radiator/heater hoses, new spark plugs, used but good distrubutor cap & rotor, one used (long) spark plug wire, solderless connectors, spare wire, fuses and spare fusable link. The fluids I put in a square 4 gallon bucket so they don't make a mess whe they leak. The tools are in a soft tool bag, and the parts are in a small plastic box. I have a set of jumper cables that I put in the truck for long trips.
Oh, I've put 50K miles on the truck since I've owned it (142 K to 195K) and I've never really had to use any of this stuff. Perpare for trips by doing good preventive maintenance at home...