The short story on SiteAdvisor is that it gives you the dirt on more than a million Web sites. Its test bots constantly check sites to see whether they send unsolicited e-mail, bury users under a blanket of pop-ups, and--perhaps most importantly--contain software that could compromise your privacy. Whenever you visit a URL, SiteAdvisor's browser icon flashes one of three colors: green (safe), yellow (caution), or red (extreme caution). Then, with a couple clicks, you can head to SiteAdvisor's home page for more detailed information. According to the company, about 90 percent of the sites tested so far have scored a green rating, whereas only 5 percent haved earned the red flag for bad behavior. My favorite part about the plug-in is that it rates sites directly from search engines such as Yahoo and Google, which reduces your chances of falling victim to a drive-by install.
You must be running Firefox in order to install it.