rel=canonical is a way to tell Google which the primary version of a URL is. It's handy if you have substantially the same content on several URLs - perhaps because you have a shopping site and allow users to sort a list of products by price or name, and this is reflected in the URL.

Adding this meta tag used to work only on the same domain.

But Google has announced today that it will support rel=canonical across domains - ie if you have the same content on more than one website, you can tell Google which is the main version you'd like it to index.

Can you use rel=canonical to fix duplicate content problems in wordpress caused by paged comments / comment pagination?