I just added tags to a view of my posts - kinda fun. Now I can find some things quicker.
Libraries can use tags in the library's catalog and on their website - in things like recommended lists, book clubs' readings. More and more people love participating. They like to add comments, reviews, suggestions. Then there is a need for organization.
del.icio.us gives you lots of ideas for things you might be interested in. When I clicked on the title of a bookmarked page, I went right to the page. When I clicked on a tag, I went to a list of sites and there was a list of related tags too. I love having all these suggestions. If you click on the saved by xxx people link, you get even more - everyone's bookmarks and you click on a person and get more bookmarks. It's lots of fun.