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SAN FRANCISCO -- Web surfers can now get a little free advice on the trustworthiness of the sites they visit, thanks to a new browser plug-in released Wednesday by Boston's SiteAdvisor.

The SiteAdvisor software, downloadable here, works with Internet Explorer and Firefox browsers, and draws on information compiled from millions of automated Web site visits to let users know whether visiting these sites is likely to yield annoyances such as spam, spyware, or computer viruses.

How It Works

To warn users that they are visiting an untrustworthy Web site, SiteAdvisor places a red X in the bottom corner of the browser. A yellow exclamation mark means that users should be cautious because tests have revealed some issues with the site, and a green checkmark means that the site is trustworthy.

With Google, MSN, and Yahoo search pages, SiteAdvisor actually superimposes its ratings icons on top of items in the search results, making it easy for users to see when they may be on the verge of linking to a dubious Web site.

SiteAdvisor believes that Web surfers unwittingly make more than 1 billion visits to untrustworthy "red" sites every month.

The company's database doesn't include data on every Internet site, but it covers sites that account for about 95 percent of all Web traffic, SiteAdvisor said.

Free Plug-In

Currently, SiteAdvisor's plug-ins are available for free, and the company plans to deliver a premium version of the product by September, said chief technology officer Doug Wyatt. He could not say what the company expects to charge for the premium SiteAdvisor or describe its features, saying that details are still being worked out.

With spam and spyware registering among the chief annoyances facing computer users, SiteAdvisor believes that its simple interface will go a long way toward preventing people from inadvertently disclosing their e-mail addresses to spammers or getting infected with unwanted software.