Chlorine dioxide is used in an increasing number of water treatment plants, and is rapidly becoming the favorite for camping, travel water treatment.

ClO2 is a gas, light sensitive, and extremely unstable--it can explode when compressed or exposed to light. This makes in nearly impossible to transport .
It is generated by adding two different stable powders to the water.

ClO2 is always generated just before use, usually as a solution.

The powders are available for drinking water treatment compressed in tablets in the correct proportions. Or in two separate containers to be mixed with water to kill biological contaminants. It kills everything: viruses to bacteria to larger plasmids like malaria and other parasites.It breaks down into calcium or potassium chlorate.

More info:
http://en.wikipedia.org/wiki/Chlorine_dioxide or google "ClO2"

As I mentioned in my PM, it doesn't leave residual in the water, which is an issue with pools. If it can kill Giardia or Cryptosporidium, which I assume it does or it wouldn't be of much use in the tablets, it could have a place in pool sanitation.

Welcome to TFP!

If chlorine dioxide wasn't so sensitive to breakdown from sunlight, it would be a good alternative to chlorine as it is not only more effective against protozoan oocysts such as Giardia and Cryptosporidium, but it produces fewer disinfection by-products which is a primary reason it is used as an initial (but not residual) disinfectant in water treatment. It could find use to shock a pool overnight after a Crypto outbreak since it isn't affected by CYA levels so would only take 1 ppm overnight (about 9 hours) and could also possibly be useful in indoor pools. However, for use in pools it would need to go through rigorous chemical safety tests and EPA DIS/TSS-12 which would be costly.

It is interesting to note that some tablet-based water purification systems (e.g. Katadyn Micropur) use chlorine dioxide generated from chlorine oxidation of chlorite where the tablets are a combination of Dichlor and sodium chlorite. For pools, part of the problem is similar to ozone in that chlorine dioxide is in some sense too reactive so it is more difficult to maintain a residual (as Jason wrote) though in applications with no UV from sunlight, it is a bit surprising no one has gone through the process of getting a chlorine dioxide product approved for pools.