The BBC has a prototype tagging system for BBC news:

We have built a social bookmarking tool just for BBC News that allows logged in users to tag/bookmark stories and view related stories that other users have tagged using similar terms.

If you go to any story from the front page and login as 'guest'/'guest' then you can start tagging stories and see how other people have tagged the same story.

The protoype uses XML-HTTPRequest to allow users to add tags without leaving the page and it also updates the related stories box based on applied tags so that users can see what others are doing in almost real time.