All you need to do is place the scented candle in your car's cup holder. The candle wax melts when exposed to heat — such as hot weather, or your car's heater in the winter — filling your car with your favorite scents. This works best if you have a candle that comes inside a glass jar because you don't want wax getting all over the cupholder. If you have a scented candle that doesn't come in a jar, just place it in a mason jar for the same effect.

— Read more in How To Turn A Leftover Scented Candle Into A Car Deodorizer at Slash Gear.

If tossing bits of spent candles makes you feel guilty, there are ways to make your candle-buying habits more eco-friendly, like avoiding petroleum-based paraffins in favor of beeswax or soy, which have lower carbon footprints. But you can go one step further by recycling your candle scraps to make entirely new candles. The process is especially handy for making citronella candles to repel bugs naturally when you spend time outside.

— Read more in How to Frankenstein old wax scraps into a totally new candle at Popular Science.