Two engineering students at George Mason University harnessed sound to put out fires. Their chemical-free, water-free extinguisher uses sound waves to separate burning fuel from oxygen. Fire dies out without oxygen, so using the sound wave extinguisher on a small fire snuffs it out.
Researchers previously patented this idea but this is the first sound wave extinguisher to work reliably. Not only does it work, but the final prototype is handheld (though, admittedly large) and only cost $600 to build.