Like many women, my hands get really cold in winter. I often carry hand warmers with me, but I am hesitant to use them except in the most dire situations for a couple of reasons. First, once open they are basically instant trash; and second, cracking open a disposable handwarmer feels like throwing money out the window.
Wonder Warmers provide another option. These hand warmers are reusable, and contain non-toxic re-activatable ingredients (inside is a salt solution and a small steel disk). To activate the warmer, you click the disk back and forth a couple of times and the salt solution crystallizes releasing heat in the process. Once they're spent and you're home, you wrap the pouch in an old sock or some other piece of cloth and boil it for about 10 minutes or until the inside solution is totally clear and it's ready for reuse.
Wonder Warmers are not perfect: the pouch that holds the salt solution is PVC, they stay warm for 30 minutes to a couple of hours depending on if you insulate them inside a glove or jacket, but they don't stay warm all day. You do have to boil them--microwaving is not an option. And they are heavier than single use handwarmers.
But each set of warmers can be reused about 100 times before the pouch fails. That's a lot of snowshoe/ski days. Other great use for these warmers: keeping your camera or video battery juiced up in sub zero temps (stick warmer in your battery pouch or put both in your pocket), and crammed next to your hydration valve, Wonder Warmers will keep your water flowing longer, even if you forget to blow back into the hose once you've taken a sip.
Wonder Warmers come a variety of sizes. The best for backpacking are the pair of small handwarmers (3 in. x 3.5 in) that sell for $14.99. Buy them at your local retailer or at wonderwarmers.com.