Google on Tuesday introduced a new online graphics tool called Building Maker that allows users to create 3D buildings and place them in Google Earth.Having already made billions by leveraging the links created by Web site owners to power its search engine past rivals, Google has reason to continue panning for gold in the river of free content that is the Internet.

Google of course is not alone in this: Pretty much any social Web site worth anything relies on aggregating the product of free labor -- comments, reviews, ratings, clicks, and the like. The goal is to amass something of value from information that on its own has so little value that its creators give it away gladly, particularly in return for software or a service.

Some companies like online movie ticket service Fandango pursue user-generated content with the ardency of a stalker, sending e-mail solicitations after you've seen a movie to ask you to submit a review or rating. The messages might as well say, "Please write for us for free! We can monetize your work."

Google has pursued this strategy with services like Knol, SearchWiki, and various contests. And its latest gambit along these lines is Building Maker, which Google hopes to see used to add a third dimension to the mostly flat world of Google Earth.

Building with Google Building Maker is as simple as can be for a 3D application: It involves aligning squares and rectangles with existing building photographs, images which will become the texture of the extruded buildings. The technology clearly comes from Google's SketchUp modeling program, which was updated last month to version 7.1.

It would of course be rather expensive to hire 3D modelers to render the world's cities properly. So Google is outsourcing the task to its free labor pool, the Internet community.

As Google product manager Mark Limber and Google user experience designer Matt Simpson put it in a blog post, "Building Maker is the latest example of our efforts to provide users with the tools to contribute to the world's most comprehensive 3D atlas." It's the latest example of Google's efforts to encourage users to contribute to Google.

Now that's not necessarily a bad deal for those who don't mind giving their time and effort to enrich Google and Google Earth. Everyone using Google Earth will benefit from a better product that more accurately represents the world. And you can be sure that Google appreciates your contribution to the cause.