I have found Del.icio.us to be about the most useful tool there is. I work on lots of different computers and have many different personal and professional projects going all the time. It is just great to be able to assign somewhat random -- or at least unrelated -- tags to a website and then find that website again later, no matter what computer I'm on.
I recently heard an author talk about writing a book and tracking all her sources, chapter by chapter, on Del.icio.us as she was doing the research. Great idea for anyone doing research and wondering how to keep track of so many possible resources on the web.
I also like the social aspect of this -- to see who else has tagged an item; what tags they used; what their del.icio.us account looks like.
I would like to know more about libraries that allow tagging in their catalogs -- that seems like a great idea since library subject terms are pretty rigid and very specific. I think del.icio.us would also be useful in college courses or high school classes where students/teachers could share resources.