Speaking of folksonomies, I would like to mention that the very first time I came across this group tagging phenomenon, it was not at del.icio.us or flickr, it was at the Gimp Savvy Copyright-Free Photo Archive where they have selections of unindexed images and invite users of the site to help classify them. Just by looking at their list of tags, which they call master keys, you can see how this works and does not work. In specific, see this example.