Google has released what they call Google Search Wiki. Search Wiki is essentially the ability of logged-in users to add comments to search results, to be able to promote certain links to the top of the results for a given search, and to add your own URLs to a given search.
It is unclear whether Google has since withdrawn Search Wiki or if it is not yet universally available, because Search Wiki is not available to all users at this point.
If comments and link promotion eventually become a standard element of search results, logged in or not, or if all comments are visible to all users, then these features will have a profound impact on search marketing and online reputation management.
Here’s the Google video about the features: