16th February 2009
As explained succinctly by Matt Cutts from Google, a new link element called the Canonical Link Element now allows webmasters and Search Engine Optimisers the ability to consolidate duplicate content on a website into a single preferred URL, without the need for 301 redirects. This new link element is now supported by Yahoo!, MSN and Ask too, making it a new standard in web development and search engine optimisation.
The Official Google Webmaster Blog also has more Canonical Link Element information available.