Google wants to make it easier for marketers to pick appropriate Web sites for their online ads with a new tool it announced Tuesday called Ad Planner.

The tool, first reported about by The New York Times on Monday, is aimed at advertising agencies' media planners, whose job is to decide where to place their clients' ads.

By feeding Ad Planner with their target audience's demographic information, media planners will get a list of sites that should prove effective marketing vehicles.

"You can drill down further to get more detail like demographics and related searches for a particular site, or you can get aggregate statistics for the sites you've added to your media plan," reads the official blog posting.

Ad Planner data can be exported as .csv files to spreadsheet applications or to Google's own DoubleClick MediaVisor ad campaign management tool.

For now, those interested in using Ad Planner need to submit a request to Google.

Last week, Google announced a similar service called Google Trends for Web sites that is aimed at a general audience.

Since news of Ad Planner emerged on Monday, it has been suggested that Google is moving into the Web measurement market to compete against companies like comScore, Nielsen Online, Hitwise and Quantcast.

The development of Ad Planner is a natural step in Google's efforts to provide more tools and services to its advertisers, said industry analyst Greg Sterling of Sterling Market Intelligence.

"It's an evolutionary thing. It's Google bringing more targeting capabilities and more information to marketers and media buyers," Sterling said.

In particular, Ad Planner seems intended to help Google expand from its core base of search engine advertisers to large, brand marketers, he said.

For IDC analyst Karsten Weide, the Ad Planner announcement isn't particularly surprising or earth-shattering.

Big online ad companies like Google and Yahoo have failed to provide their clients with comprehensive suites of complementary ad services, for things like media planning, ad creation and ad testing, Weide said.

Tools like Ad Planner are an attempt -- in this case by Google -- to fill that gap, he said. "This is one step from Google to provide one piece of the puzzle to clients, in media planning," Weide said.

Quantcast, one of the companies Google will compete against in this market, said that, unlike Google's, its audience discovery platform is "collaborative and open" and puts publishers and marketers in control of their audiences and data.

"Of course, Google controls an extensive data platform -- and the market must ask the question if this new product is simply intended to help Google sell inventory and a broader set of controlled services," Quantcast CEO Konrad Feldman said in a statement.

ComScore, another company in this market, saw its shares tumble almost 23 percent to $21.45 on Tuesday, a Wall Street reaction no doubt influenced by Google's announcement.

When asked for further comment about Ad Planner, a Google spokesman said no one at the company was available to talk on Tuesday.