My hat goes off to the researchers at Stellenbosch University in South Africa, who developed this nano-fiber-filled tea bag. It can safely filter a liter of water, and it costs less than a cent.
The filter itself isn't reusable—or ready for mass production—and even at a fraction of a cent, it could be too expensive for those who need it most. However, it does have the potential to help deliver potable water in a scalable way.
In the following video you can see how it would work in conjunction with a reusable water bottle.