Quantcast is a new service allowing for an open internet ratings service. From doing a bit of research, Quantcast seems to be the only service of its kind providing such a service for free.

Thier ratings service already has hundreds of thousands of popular destinations indexed, creating a common language for publishers and advertisers to discuss audiences and advertising opportunties.

You can read about how they obtain their estimates for ratings or a bit more about how they analyze their panels, an analysis of internet visits from a set of panel members about which certain demographic information is known.

What's cool about Quantcast is that metrics gathered are free and you can even add your own web site to their system to help improve their ratings tool.

For advertisers, Quantcast can "provid[e] extensive coverage of internet destinations and consistent audience reporting, we want to make it easier for you to find and research audiences for your campaigns." For publishers, the service helps to understand your traffic in realtime and to build higher advertising yields by segmenting your audience.