Tagging

I can see how tagging a site would help you stay organized, help others to find a site they might like, and also help the site itself with increased traffic. On one of the blogs I go to regularly, I've seen her thank someone for their tag. Now I know what she was talking about. I'm guessing that delicious tells the site when they've been tagged.
This would definitely help with research. For home schooling, for example, there are tons of wonderful websites with free games. I quickly accumulated so many that they became useless. Just going by the little bit of bookmark name is not enough. With tagging I can organize by "multiplication" or "writing_organizers" or "vocabulary." Then I can see what others have tagged as "vocabulary."
I also love the fact that all of this work organizing is not limited to one computer, but can be accessed anywhere you can remember your username and password.