Google, Microsoft, and Yahoo have added support for the new Canonical Link element to help identify a link to content on your site where all traffic should land when more than one link exists to the same content. The Canonical Link will be used for the search engine index, and sitemap URLs will still be used when there is a tie between more than one URL link.
WordPress 2.9 has now integrated the rel=”canonical” link element, which will display between the “head” tags, but only on single post pages:

Note: WordPress itself does not actually integrate the wp.me short url format in the core. The Stats plugin, version 1.6 or above, is required for the wp.me short url format to be integrated.
Drupal has not added core support for the rel=”canonical” link element, so Yoast has created a module that will add this feature to Drupal.
If you’re using WordPress, as NG does, then you will want to have the rel=”canonical” link element on all the pages of you’re blog such as the homepage, tag, content, and other pages of your blog, so that each page can tell the search engines which link to use for all similar content that might otherwise be labeled as duplicate content. For instance, if you use the Google analytics campaign tag for traffic coming from Twitter your link might look like:

For this page the Canonical link should be:

To make sure all your pages tell the search engine which link to use you’ll need a plugin to help with this SEO (search engine optimization). Here at NerdGrind we use the a WordPress plugin called All in One SEO Pack, which has included Canonical URLs for a while. The All in One SEO Pack is highly recommended.
The addition of Canonical Links can significantly improve the SEO of your blog, and this can result in more traffic, as well as higher quality traffic.