Link ‘canonical’ tag

This tag is applied to the page to define the preferred URL to be stored in most search engines, and reduces the odds that heuristics will be used and leave you will multiple listings of duplicated content (example: http://www.example.com/ vs. http://example.com/)

It can also be applied so that the URL’s exposed by search engine results do not expose the underlying technology such as PHP/JSP/ASP in the filenames)
<link rel="canonical" href="http://www.giantgeek.com/" />