Thought this was an interesting discussion on using the filesystem (and lots of symbolic links) for “tagging” in the http://del.icio.us sense of the word : http://blueslugs.com/wordpress/index.php/archives/2005/07/12/tag1-delicious-style-file-tagging/
Might be an interesting experimental feature for dokuwiki
You can use namespaces to create a crude tagging system. Create a link on the page you want to tag something like this [[tag:tag]] (tag).Clicking on the title of that page and viewing the backlinks is the equivalent of an index of all the pages you have tagged with that tag. It only works for single tags but it should be possible to write a plugin to extend this for searching on more than one tag. Backlinks work even if the page hasn't been created yet, but it is useful to create it, even if its blank, so it shows up under tags in the index. I use the tag pages to store links to other tags so that tags can be grouped together — you can then navigate around related tags by clicking on the links. — Paul M 2006-01-20 18:37
The blog plug-in already has tags available.
But the tagged article list of a tag has still to be added manually, not generated automatically as MediaWiki does. A built-in tag/category feature is what I miss most for DokuWiki. — Roberto Ciang 2007-04-02 14:23
http://wetpaint.com host wikis and have a handy keyword tag feature - for example see bottom-left of http://www.wikifido.com/