Duct tape is small and light so it's easy to keep in the trunk and can save you in a bind for situations like keeping your muffler attached temporarily.
Honestly, once you start carrying around duct tape you'll be surprised at all the applications it has. Did someone knock off one of your side mirrors? Duct tape will work as a temporary fix until you can get it repaired. Small tear in your upholstery? Seal it up with duct tape ASAP to keep it from growing larger.
WD-40
This popular lubricant can come in handy in a pinch, helping to loosen a bolt when changing a tire or doing just about any work on your car. It's also great for removing gunk and gum that may have gotten stuck to your car.
Let's face it, you probably have a can of WD-40 somewhere in your house already, right? You may as well just keep the can in your car; it's small, lightweight, and can be a real lifesaver in an emergency situation.
Blankets
It's good to have a blanket to stay warm and comfortable in your car. This is especially important if you live in an area where it gets very cold during the Winter time; you never know when you might get stranded and have to spend the night in a broken-down car with no heater.
Even in non-emergency situation blankets can come in handy. Forgot to bring a jacket on a camping trip? Wrap yourself up in a blanket to stay warm. Friend wants to bring his muddy dog into your car? Put a blanket down on the seat to save your upholstery.
Gas Can
Running out of gas is a lot like locking your keys in your car, you don't think it's going to happen but then it does and it's embarrassing. Keep a gas can with a few gallons of gas in it in your car at all times. When you run out of gas in the middle of nowhere this could be your only hope to make it to the next gas station.
Water
If you get stuck somewhere or have to walk a far distance to get help you are going to get thirsty so don't forget to pack some water. You might also need it for your radiator or to clean your windshield.
In an emergency water will become a very important commodity, just in case you should have a gallon or more in the trunk of your car. It will weigh you down a bit, but it's definitely worth it.
Spare Key
Don't feel bad about locking your keys in your car, we've all done it before and will most likely do it again at some point. The best way to avoid this is by stashing a spare key somewhere on your car with a magnetic key holder.
The best place is underneath the bottom of a car, make it a hard to reach spot. Obviously you don't want it in an easy-to-spot place where a thief could potentially find your keys.
