one i saw that i thought was cool was a clay pot painted with a silver solution on the outside and fits into a 5 gallon bucket. water weeps through clay pot, filtering out sediment. silver solution takes care of other pathogens (bacteria, virus, etc). resulting water is clean and drinkable and the whole system is cheap.
not sure who sells them in the states, but developed for and used in a lot of poor countries.
think certain types of iodine works great for water purification.