Katadyn BeFree 0.6L Water Filter

This water purifier is nearly magical. You pack it empty, and then when you find a water source on your hike, you fill it and cap it. The water is filtered as it exits the purifier. I've used it many times, including in lakes where there's visibly all kinds of little organic things growing in the water and the water tastes like it came out of my tap. As a bonus, the water can have a mineral taste, if it's been running over rocks on it's way to your source.