Google recently launched a service where they track flu related queries that people are using in their searches. Analysing their data back to 2003 and comparing it to the CDC network they found that they can pinpoint the start of a flu epidemic two weeks earlier than the CDC. They can even follow the spread of the flu geographically (at least, in the USA).
This is I think an amazing example of how we can use this kind of implicit “wisdom of crowds” type of ideas. considering the risk of the swine flu epidemic in Mexico this might come in handy… The flow of the epidemic can be followed in Google maps.