What’s nice about Font Fonter is that while previewing a website in a different font, you can go any levels deep and the pages will still render in the previously selected typeface.
This helps when sites use different styles for the home page and inner pages or when you are a search page and are navigating to a new page that could be using a completely different typeface – your font selection will be preserved during the entire session.
You need Firefox or Chrome to use Font Fonter, it simply won't work in IE and Opera. FontsLive, another font vendor, also offers a similar preview service where you can again replace the default font styles of a web page with premium web fonts.
Preview Web Pages in Google Fonts