Last week Microsoft, Google, and Yahoo came to an agreement on a new link element that will help clean up duplicate URLS of sites.  What does that mean?

For the general public this will mean less duplication during searches.
Web publishers will like this as Canonical Links will condense all incoming traffic to a single site URL. 

How this works:
A link similar to:
<link rel=”canonical” href=”http://example.com/page.html”/>
is inserted into the head section of web pages. Regardless of the URL a search engine uses to visit a web page the one indicated in the canonical link is the URL that will be indexed.

Concerns:
Pages may use the Canonical Link to direct traffic to another page or site in hopes of increasing page rank or other standing.

You can read more about it on Matt Cutts Blog, Google Software Engineer.

You can also find implementations for popular blogging and content management tools here.