Google is launching a new service designed to let website publishers build their own search engines using Google's massive index of page links.

The Google Custom Search Engine service will let individuals or organisations put a Google-powered search box on their websites that only searches certain sites and pages. That way, the publisher of a website about technology, for example, could put a search box on their home page that only returns links to pages about that category. The service will also let publishers have a search engine that taps Google's index in full but gives preference to results from websites they have pre-selected.

With Custom Search Engine, Google joins others that provide similar services, including Yahoo and Rollyo. These custom engines are part of the social search concept, which taps users to refine the search engine experience by contributing, categorising, tagging and sharing search results. For example, site publishers using the Google service can let others contribute to their custom index.

Designed to be extremely easy and intuitive to use, the service will let site publishers build their own search engines in a matter of minutes through the use of menus and wizards, said Marissa Mayer, Google's vice-president of search products and user experience.

The service is hosted on Google servers, so site publishers don't get access to query logs, a sensitive topic for those concerned about the privacy of their search activities. However, the entire process happens in the publisher's site, and they can personalise the search results page so that its layout is in tune with the rest of their pages, Mayer said. Google will display contextual adverts with search results, but sites run by government agencies, non-profit organisations and universities can opt out of this.

Google stands to benefit not only from sharing advertising revenue with publishers but also from propagating the availability of its search engine, said Greg Sterling, an analyst at Sterling Market Intelligence. "People are doing searches on all kinds of websites, not just search engines," he said. "Search has become the web's navigational paradigm."

Meanwhile, there is considerable demand from publishers for custom search engines because they realise that providing that capability makes their websites more attractive to their visitors, Sterling said.

The new service is now available here.