Matt Cutts of Google introduces the canonical link element.

Resources:
Blog post on Google webmaster blog:
http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

Yahoo blog post:
http://ysearchblog.com/2009/02/12/fighting-duplication-adding-more-arrows-to-your-quiver/

Microsoft:
http://blogs.msdn.com/webmaster/archive/2009/02/12/partnering-to-help-solve-duplicate-content-issues.aspx

Ask:
http://blog.ask.com/2009/02/ask-is-going-canonical.html

Google Help Center documentation:
http://google.com/support/webmasters/bin/answer.py?answer=139394

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