Of interest to Blog users is the new tag indexing service on Technorati. Technorati has started gathering tagged results from Flickr and del.icio.us and includes them in their index so you can search, for instance tag:nyc and retrieve all entries using that user supplied tag on those systems.
Blog systems that support categories can use the following elements, which technorati will index as tags:
I'm not sure what technorati will do with categories that have spaces in them, however, so that makes the use of existing categories questionable. It would be nice if they remove the spaces in those categories or something.
If your web publishing software doesn't support categories or if you don't know how to configure them to add categories to your RSS feed, you can include a link that identifies your entry with a tag and technorati will index it, e.g. <a href="http://technorati.com/tag/[tagname]" rel="tag">[tagname]</a>. This also seems particularly useful for tagging wikis or manually created html pages.
If this indexing takes off as a system for aggregating topics across different sites, I can imagine that a lot of people might start to become interested in creating controlled tag lists. If the tags are unique enough then this might be a usable system until someone creates an service that is solely dedicated to this type of cross-site topic mapping.
Here's an example of how you might hack together a process to use on Technorati. Create a tag set that prepends each tag with a long string followed by the tag, e.g. drupal-bloggers-[tag], and get everyone in your community to start using that tag as their category. Make sure your blog system puts out the category in RSS or Atom feeds. Then every term that uses that tag will be aggregated in Technorati. Problem is that Technorati is not providing an XML feed for the results, so you can't actually aggregate them in your own RSS reader. They do have an API, however, so I'm sure someone could create an application that takes advantage of this tag indexing. I wonder how long it will take before Feedster does.
So here's the open question to Drupal developers. Can we get categories in our RSS feeds? We Drupal users already have RSS feeds by taxonomy (e.g. at the bottom of any of my category pages there is an XML feed button), but it would be nice if we can get our categories indexed by Technorati as well. I'd love to do a proof of concept for community tagging around a topic. The participants could create a community-editable page to manage the master list of tags to use for something like Drupal related topics. Then we get our blogs to output tags in RSS.