A LED flash light and fold out triangle reflector. Even a small flashing red light may help others see you and your car. Some folks use the "shake up" or "wind up" variety, to avoid dead batteries.

A car blanket is also a good item to have onboard. If one gets in an accident, it can help reduce the impacta of shock.

I also carry a small spring loaded impact device (found in hardware stores) to assist in breaking a side window, should my vehicle slip offroad into a river or lake.