Bug and Dragon
I don't know what the most popular browser SEO Freelancers use, but Firefox maniacs (as me) love SEO Firefox plugins, especially beloved Firebug - a Firefox important Web Developers Add-on (created by Joe Hewitt in 2006), which is now free and open source, and available for IE, Opera and Safari.
Sooth to say, I never heard of any Web Developer with Internet Explorer Developer Toolbar. But you?
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