Until I saw this Wired post, I had not heard anything about Dean Kamen's new "Slingshot" technology, which purifies bad water. It seems promising. According to Wired, Kamen's goal is to bring down the cost down to $1,000-$2,000 per unit.
It appears to me part of a growing trend to get cheap, portable, and powerful technology into the hands of the world's poor. Examples include the $100 laptop and this low cost wind power generator. Instapundit posts on the topic here and Popular Mechanics focuses on it in this podcast.
Hopefully, this is just the beginning of the trend.