Last year, Michael Pritchard did a TED Talk which was incredible. His idea, a product called the Lifesaver filter. (See the TED Talk here
Basically, this filter uses extremely small (15nm) filters to block even the smallest of bacteria or viruses. The bottle itself is extremely easy to use, and creates completely clean drinking water from anything as toxic and vile as toxic waste within seconds.
According to Michael Pritchard, each bottle is good for 6,000 L before a small cartridge needs to be replaced.
The technology is also in place in Jerry Cans. These Jerry Cans are good for 25,000 L, apparently good enough for a family of four to survive for three years.
How much would you guess this technology would cost? It sounds like it may be expensive but, according to Michael Pritchard, it would cost a family no more than half of a cent per day.
EDIT: Apparently the prices have been raised since the TED Talk was given. The current prices make the bottles and Jerry Cans almost useless for anyone in a third world country. The bottles (which last for 6,000 L) are about $200 US, with $100 US refills. The Jerry Cans are $400 for 20,000 L.
If there is more research done on this technology which makes it cheaper to both manufacture and sell, this could be of great benefit. Until then, we still need to look for other solutions to the water crisis.