Steripen Freedom

Cost: $120

You never know when you might have a day like Bear Grylls': stuck in the middle of nowhere and dependent on the local water supply. This economical water-purification system uses ultraviolet light to turn contaminated water into potable water (as long as the contamination is something like bacteria, viral agents, or protozoa and not, say, uranium). To purify water, you stir using the pen for about a minute until the tip turns green. The pen weighs only 2.6 ounces, so you can keep it handy in a backpack, and you can charge it using a standard USB cable. The bulb lasts for 8000 uses, or once per day for the next 21 years.