Jumpers, spare tire, factory jack, 4-way lug wrench, coolant, spare (old) serpentine, oil, coolant, socket set, a few screwdrivers and wrenches, blanket, knife, some cash, and a CB radio. Snow shovel in the winter.

Not really for emergencies, but I usually have an extra oil filter in the same box in the back. I don't know why I keep it there instead of the garage, but I do.