I've used the WOT (Web Of Trust) extension for Firefox for over a year now and it's been very effective. You'd be surprised how how easy it is to be fooled by legitimate looking websites. The WOT extension monitors links and automatically blocks sites that are considered dangerous. You can override the blocked website on an individual basis. It also provides ratings on search results and Wikipedia links. They recently also published a list of the Top 50 Dangerous Websites.
WOT Home page
In a related item, the McAfee Site Advisor has a spam quiz. Can you tell which sites are safe and which ones will sell your email address? Try it here.
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