And more: http://tagschema.com/blogs/tagschema/
OOh -- something that's been bugging me for a long time! (http://www.pui.ch/phred/archives/2006/02/delicious-vs-rawsugar-vs-simpy.html )
Feature in RawSugar -- "They have got /hierarchic tags/, and this is a very clever idea. First I wasn’t totally convinced about this feature but it has itself proved to be very helpful. I don’t like del.icio.us’ tag bundles. Tag bundles may be a good thing if you have 100 to 200 tags, but if you have 500 or 1000 tags (I currently have over 2000), it isn’t manageable any more. With RawSugar you can put a hierarchy in your tags while tagging an item. if you type “article>rant” you say that “rant” is a child tag to “article”. If you later tag a site with “rant”, the tag “article” will automatically be added."
A nice rant on what tags are good for, and what it's assumed they're good for, and such -- http://www.pui.ch/phred/archives/2005/11/how-tagging-could-gain-ground.html
broad versus narrow folksonomies: http://www.personalinfocloud.com/2005/02/explaining_and_.html (I thought I'd understood this until I went through the examples, which I didn't really follow; although then, from the comments, maybe I did..)
reviews of a lot of bookmark-tagging sites: http://3spots.blogspot.com/2006/01/all-social-that-can-bookmark.html
and wow, thorough matrix of them linked from here: http://www.roxomatic.de/archives/985/Social-bookmarks-review-version-3.5
and after all of that, http://www.pui.ch/phred/archives/2005/07/analyzing-tag-connections.html is really good (and yeah, I know more than half of these are articles from the first site I posted, but I figure this lessens the energy of activation for looking at them)
Okay, I'm done with tagging links for a bit.
Last thing, for now, is to investigate the worth of using FreeTag -- http://www.getluky.net/freetag/