Here’s a neat Firefox extension: SiteAdvisor. Watches what you’re browsing, throws an alert up if you’re looking at something sketchy — with malware, spam, phishing, etc. Also watches Google search results & proactively shows links that might give you trouble.
This is an interesting trend in new extensions for Firefox — lots of out-of-band information about what you’re looking at. First pioneered a while back by Alexa, I suppose. I particularly like the user experience of siteadvisor.