I was reading an article in the Washington Post about Wikiality (the idea that if enough Wikipedia readers believe that something then it must be true) and it mentioned a service called cumul.us described as "a wiki-weather site in which users can collaboratively decide whether it is raining outside". That description is not very far from the truth.
The idea is that the aggregation of information in groups results in decisions that are often better than could have been made by any single member of the group.
cumul.us is built under the premise that aggregating as many available forecasts as possible (both from experts and not) will result in a more accurate forecast, and that the aggregation will be a new source in itself.
Their faith in the massed ranks of anonymous users is quite touching.