For those of you who don't know tagging is a little bit of a phenomenon sweeping the web right now. The idea is that any content can be "tagged" with a series of words which will describe it and then you can reaccess that content through the words. Tagging leads to tag clouds where the tags for a piece of content or across a community of users are displayed in a paragraph and the font size corresponds to the tag popularity. Tagging also leads to tag search engines, community categorization and loads more interesting applications.
To my mind however tagging is a very old thing in terms of the web. Tagging is (IMHO) a way of attaching the most relevant keyword/keywords to a set of content. Effectively anchor text ("<a href='someurlorother'>THIS IS ANCHOR TEXT</a>") is just a form of "tagging" by webmasters and titles for webpages are just strings of tags the owners have put together.
Putting together these existing "tags" and the concept of tagging has however been giving me some interesting ideas. Auction sites, classified sites and personal sites effectively receive thousands of tags a day. Self learning category structures could be created really powerfully from this stream of user generated tags and since it is in the interest of the millions of these users to describe their products or items for sale so they sell well it is definitely in their interest to make the tags as relevant and accurate as possible to the ideal (if not always the actual) product. All this data is exposed to any API programmer and someone with the skills and interest could create a product search more powerful than any of the Kelkoo's etc... out there right now with millions of small businesses and individuals automatically helping them with accuracy without even realizing it.
I suspect I may post more on this later since I am now planning to give it a go in a small category of listings on eBay. My previous post on temporal perturbation definitely has relevance in this area too.