Official Google Webmaster Central Blog: Specify your canonical
http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

Google's way of allowing sites to specify a normalized URL form for their pages. This allows google to treat various forms of the same URL as one URL for search purposes.

Carpe diem on any duplicate content worries: we now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.

webmaster central

HTML に、<link rel="canonical" href="http://mysite.com"/> を加える、と。

ページネーション使ってるページとかにも使っていいのかな。

If tags conflict (such as pages point to each other as canonical, the URL specified as canonical redirects to a non-canonical version, or the page specified as canonical doesn’t exist), search engines will sort things out just as they do now, and will determine which URL they think is the best canonical version.

Specify the canonical version using a tag in the head section of the page as follows: <link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish"/> That’s it!

Canonical explanation.

Last week Google, Yahoo, and Microsoft announced support for a new link element to clean up duplicate urls on sites. The syntax is pretty simple: An ugly url such as http://www.example.com/page.html?sid=asdf314159265 can specify in the HEAD part of the document the following: <link rel="canonical" href="http://example.com/page.html"/>

"RevCanonical is url shortening with a twist. Instead of creating its own super short versions of links, it checks to see if the link owner has published a shortened version of the given page using HTML link element. If not, we just return the original URL. And you should bug the link owner about providing a better alternative."

"url shortening with a twist. Instead of creating its own super short versions of links, it checks to see if the link owner has published a shortened version of the given page using HTML link element. If not, we just return the original URL. And you should bug the link owner about providing a better alternative."

A URL-shortening bookmarklet that looks for a canonical shortened URL (for example, a http://flic.kr link for a Flickr photo, or a gsfn.us link for Get Satisfaction) and returns it, instead of using a third-party service.

RevCanonical is url shortening with a twist. Instead of creating its own super short versions of links, it checks to see if the link owner has published a shortened version of the given page using HTML link element. If not, we just return the original URL. And you should bug the link owner about providing a better alternative.

Bookmarklet that will 1st check canonical, if there is no shortURL, then it will create one using TinyURL

from simon willison's weblog.

A bookmarklet to extract rev="canonical" links from pages. I only use a few sites that support this at the moment (notably Flickr), but it's quite handy.

django-shorturls

……（他のhead要素）…… これは、ヤフー、MSN、グーグルに対し、そのページを「www.seomoz.org/blog」というURLのコピーとして扱うべきだということと、検索エンジンが適用するリンクとコンテンツの指標はすべてこのURLに還元すべきだということを伝えるものだ。