Chlorine in water
An army friend of mine said he used to carry a small eyedropper bottle full of straight chlorine bleach. He get his drinking water from a clear stream and would squeeze one drop of bleach in his canteen for clear water and two drops if it was cloudy.
He said that since straight bleach is so highly concentrated that it would kill the bacteria in the water, and such a small amount would not have any detrimental effect on the human body.
I haven't been able to confirm this as a viable water purification method.