Google has just released a new extension for Firefox, Blogger Web Comments, which shows a small (configurable) popup showing blog articles (which they get from Google Blog Search) that mention the page you’re currently viewing.
I’ve only been using it for 3 minutes or so, so I still haven’t thought about the implications of, basically, for each page you browse to, getting a list of blog articles talking about it. I think it could actually change the browsing habits of a lot of people. A reasonable way to put it might be: “what if every single page in the world had easily accessible comments - and not controlled by the page’s owner?”
It could become annoying, though - I’ll have to wait and see. It can be hidden or temporarily disabled, of course. Or uninstalled, naturally - this isn’t Internet Explorer. 
(seen on ProBlogger)
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Yeah, that’s been tried before - remember “Third Voice”?