WordPress.org

This simple plugin helps you easily tell the search engine bots the preferred version of a page by specifying the canonical properly within your head tag.

Why would you do such a thing? Simply because many black-hat SEO folks abuse duplicate content to gain search ranking and search engine try really hard to detect and penalize them. The last thing you want to do is present yourself as an SEO spammer to search engines.

WordPress 2.9+ includes rel="canonical" header settings by default but the feature only works properly on singular posts (not category or archive pages). So for example:

The second page of your homepage should actually have its canonical URL set to: "http://example.com" instead of "http://example.com/page/2"

Using this plugin solves the above issue in WordPress 2.9+.