Mylar blanket and/or wool blanket
Winter bonus items:
These might not be all that useful year-round, but depending on the climate of your home (or along your road-trip route), these can be life-savers--literally--in the winter.
The mylar blanket relies on its excellent reflectivity to trap your body heat and keep you warm, so be sure to wrap it completely around you. Even for those not traveling in or through especially cold climates, the mylar can be helpful, as it also keeps the heat of the sun out, and can function as an emergency reflector if you need to signal someone from a distance.
A wool blanket (or other fabric blanket) is a great choice for its warmth-preserving properties, but it also can have many other uses. Bundled, it can serve as a pillow or other cushion. Cut into strips with your knife, it can serve as a bandage or sling. The uses are almost endless if you're creative--and necessity is, after all, the mother of invention.
This one is obvious, but it's something many people forget, or leave at home in the garage. Getting the ice off your windows is a vital part of safe driving in the winter, and it's made all the easier with a purpose-built tool. Don't rely on your credit card or library ID.
Whether your car is front-wheel drive, rear-wheel drive, or all-wheel drive, chains can get you out of a slippery situation when used properly. Be sure to read the instructions and your owner's manual for details on how--or if--you should use them.
Compact and often multi-faceted (some have hatchet functions or waterproof handle compartments to store items like matches), the folding shovel can be handy in many situations, too. For winter use, a shovel can dig you out of a roadside snow bank, clear a path for spinning wheels, and more.