Obiki has tagging support for documents.  When you're editing a document you'll now see a text field at the top labeled 'tags'.  You can add a comma-separated ist here (it'll start auto-completing once you save some tags... like del.icio.us).  Then you can find pages based on tags from /index.tags on the site.  If you want it to only show pages from a certain section, you would go to the parent page of that section and change
the flavour to 'tags'.  This is probably most useful for resource collections.