Sometimes the most useful gadgets are the simplest ones. Case in point: the Zippo Hand Warmer. This gizmo burns lighter fluid to prevent your fingers from freezing for up to 12 hours. Buy some refills, and you can reuse it countless times. These warmers are easy to fill, come in a variety of colors (pick from orange, black, silver, pink, or camo-patterned), and produce very little odor. Keep one in your coat or bag so you'll have it close to hand whenever you head outside.