A pressing issue in the developing world is access to clean and safe drinking water. The Copenhagen Consensus points to several water issues as among the "good projects" it identifies.
I came across this interesting article on efforts by private entrepreneurs to deliver safe drinking water in the developing world and the failures of the private sector to do so. As with many other scarce commodities, it is being argued that the problem here is a lack of well-defined property rights. An interesting, and important, challenge.