10 Mylar Emergency Blankets
Effective and inexpensive. Every car should have enough for the maximum passenger capacity during normal everyday life -- in the winter, it could save your life.
The reflective mylar reflects your body heat back, keeping you warm and dry in cold conditions. If you've ever run a marathon, they often hand this type of blanket to you at the end of the race to keep you warm because you are drenched in sweat and the slightest breeze could set on hypothermia in just 55 degree weather.
So many more uses for this item.
Line a shelter roof with one to make it both waterproof and to reflect the heat inside.
Make a semi-circle-wrapped barrier around a fire to reflect the heat back to your party.
Create a water collection resevoir.
Protect a hypothermic person.
Line the bottom of a sleeping bag.
...the list goes on.