Chalk up researching online media buys as yet another thing Google will let users do for free, according to items in the New York Times and Washington Post this afternoon. Set to be unveiled tomorrow at Advertising Research Foundation’s conference, the word leaked out when the conference put up an announcement of the new service, before yanking it back down. (As the NYT notes, the release can still be found in Google’s cache.)

The service will reportedly be similar to the recently released Google Trends for Websites, allowing media buyers to use both Google’s own internal server data as well as third-party infomation view demographic information on users across websites, helping them to target relevant ad inventory. It’s unclear how much more information AdPlanner will provide — Google Trends for Web sites currently only shows user’s country of origin, other sites visited, and common search terms.

The service will put Google in direct competition with Nielsen Online and comScore, the two other firms offering audience research information today. While both charge for their information, Google’s AdPlanner will reportedly be free of charge.

The news comes after years of frustration from advertisers and publishers with comScore and Nielsen Online’s numbers. The Internet Advertising Bureau found marked discrepencies between ComScore and Neilsen Online’s reported numbers and member websites’ own internal logs. In one case, Nielsen Online reported mlb.com drawing in 6.2 million monthly users, while mlb.com’s internal metrics showed over 19 million. Nielsen Online and ComScore have agreed to audits in 2007 by the Media Ratings Council, audits that have yet to be completed.