Web surfers can now get free advice on the trustworthiness of sites, thanks to a new browser plug-in from SiteAdvisor.

The software, which can be downloaded from the SiteAdvisor website, draws on information compiled from millions of automated website visits to let users know whether visiting these sites is likely to yield annoyances such as spam, spyware or computer viruses.

SiteAdvisor warns users when they are visiting untrustworthy websites by placing 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 the Google, MSN, and Yahoo search pages, SiteAdvisor actually superimposes its ratings icons right on top of items in the search results, making it easy for users to see when they may be on the verge of clicking on a dubious website.

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

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

The SiteAdvisor plug-ins are available for free now, 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 any of its features, saying that details are still being worked out.

With spam and spyware registering as among the major 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.