Carbon nanotubes are a substance made from carbon atoms bound together like this —
They have a lot of very interesting and very, very useful properties. For one thing, they have enormous tensile strength. A cable the thickness of a human hair made using carbon nanotubes could hold up a car. I can sense all the fly fishers reading this getting very excited at the prospect of a cable that strong, but it has other uses as well, for exmple, space elevators could be made using such cables, allowing easier access to space.
Most importantly of all, they can be used as filters to purify and desalinate water as discussed here.
Future uses of nanotubes may include providing the circuitry and motors for nanotechnology, in flywheels and capacitors to store electricity and in photovoltaics. However, even using only the applications that have been fully worked-out and realized, they have the capacity to change civilization.
They are not particularly expensive, even in a scarcity-based economy. They are made from simple carbon, which is cheap and common. There are companies selling them for $3 a gram. If production were scaled up to mass-production, the price would go down even further.