Matt Cutts of Google introduces the canonical link element.

Resources:
Blog post on Google webmaster blog:
http://googlewebmastercentral.blogspo…

Yahoo blog post:
http://ysearchblog.com/2009/02/12/fig…

Microsoft:
http://blogs.msdn.com/webmaster/archi…

Ask:
http://blog.ask.com/2009/02/ask-is-go…

Google Help Center documentation:
http://google.com/support/webmasters/…

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