Matt Cutts of Google introduces the canonical link element.

Resources:
Blog post on Google webmaster blog:
http://googlewebmastercentr...

Yahoo blog post:
http://ysearchblog.com/2009...

Microsoft:
http://blogs.msdn.com/webma...

Ask:
http://blog.ask.com/2009/02...

Google Help Center documentation:
http://google.com/support/w...

Joost de Valk: WordPress, Magento, and Drupal
http://yoast.com/canonical-...