The Lifestraw Family water filtration system is a small gravity- and hand-powered apparatus that renders dirty water drinkable. It could really improve life for the millions of our fellow humans without water sanitation. Each system costs $25 and is good for 15,000 liters, which, we're told, is two years' worth of safe drinking water. Totally worth it.
There's more about the Lifestraw Family at Inhabitat here. If you're hurting for some good karma (maybe you have a taste for expensive bottled water shipped from a pristine, far-away island) you can donate one of these filters through Project H Design (on this page, to be exact).