Google, Yahoo and Microsoft started using Canonical URL

Google, Yahoo and Microsoft have joined forces to rid their search index of duplicate content.

For reference: duplicate content - pages with the same content available at different URLs.

So, Google, Yahoo and Live Search started to support the “new” HTML tag:

  <link rel = "canonical" href = "http://example.com/products" /> 

When adding this tag, different URLs with the same content, for example:

  http://example.com/products?trackingid=feed
 http://example.com/products?sessionid=hgjkeor2
 http://example.com/products?printable=yes&trackingid=footer 

will be considered secondary and their search weight, as well as they themselves will not be taken into account when ranking the main (specified in the tag) page in the results of search engines.
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Thus, they offer webmasters to add this tag to the html-code if the architecture of their web-sites generates the possibility of accessing the same page at different URL-addresses. And help them avoid imposing sanctions due to duplicate content.

Information about this tag from the search engines themselves: Google Webmaster Blog , Yahoo Search Blog , Live Search Blog

Ps very free translation and my first post on "you know where" =)