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Yesterday, Google announced the launch of Sidewiki. Sidewiki is basically an add on to the Google Toolbar that allows you to comment about any site, directly on that site, via the Google Toolbar. For more about the tool, see the video below and Danny's detailed writeup.
SEOs are discussing how Google may be able to use this for ranking data at a WebmasterWorld thread. Plus Google is asking for feedback on the tool in the Google Toolbar Help forum.
Forum discussion at WebmasterWorld and Google Toolbar Help.
