WOT Web of Trust
Free browser plug-in Web of Trust is available for Chrome, Firefox, Internet Explorer, and Safari. This clever tool vets search results before you click them, using a traffic- light rating system (red, yellow, green) to indicate which links are safe, which you should avoid, and which require caution. And it includes crowd-sourced ratings, meaning a site's reputation is based on other users' experiences, not some potentially outdated definition file. (You can add your own ratings as you surf, thought that's entirely optional.) Keep in mind, however, that while WOT works well with browser-based mail services like Gmail and Yahoo, it won't help you with mail received in Outlook, Windows Live Mail, and other desktop clients. For that you should install some kind of local security software.
Note: This link takes you to the vendor's site, which automatically detects whether you're using Firefox or IE and offers you the correct version for your browser.