I've heard about using sun exposure to naturally UV purify water. There's an article somewhere about an individual in Africa running a business selling drinking water purified in bottles on the tin roof of their house.

But, I'd always assumed this was impractical for backpacking, unless you're going to be in a very sunny area and don't mind carrying a 12 hour supply of water (six hours worth hung on your pack in the sun and six hours worth of clean water to drink in the meantime). I suppose it would be useful for camping in the same place for a while.