Google got a lot of attention recently for the launch of Google Flu Trends, which looks at aggregate data on searches related to the flu, to see if it can act as something of an early warning system for where there are flu problems.
It's an interesting use of the data, and it will be worth watching what else can be done with this sort of data over time.
However, Ed Felten raises an interesting question: can Google Flu Trends be manipulated?