Quantcast is media measurement service that combines directly measured audience data with panel-based estimates to deliver third-party metrics on websites for advertisers or others seeking traffic data. Quantcast measures traffic both on web sites and flash media such as videos or widgets. Quantcast uses what they call a Mass Inference algorithm to produce refined profiles that they believe accurately represent web audiences. Users can see estimates for gender, age, household income, and other demographic information.

Above: Quantcast Screenshot -- #1