Google launched a new service today called "AdPlanner" which gives advertisers a clearer picture of where they should be looking to place ads online. Taking user demographics into consideration, Google will suggest a list of websites that it recommends the advertiser should place ads. This type of information can sometimes be easy to guess, but only Google knows for sure where your users are.
This tool seems to be backed by the the same technology that runs Google Trends for Websites -- according to the help files, it pulls a lot of the same data from the same sources:
Google Ad Planner combines information from a variety of sources, such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms; it doesn't contain personally-identifiable information. ... Google Trends for Websites is designed for all users. Google Ad Planner is designed for advertisers and agencies, and offers features such as .csv exports, MediaVisor exports, and demographic estimates.
The new tool is more of a research tool than anything -- you can't use it contact site owners or buy ad placement on the sites it recommends. That said, you are able to export your "Media Plan" to tools like DoubleClick MediaVisor.