Del.icio.us seems to be a great way to really organize bookmarks. I can see that it is useful for connecting to many sites. Like Flickr, you create tags, but instead of tags to photos, it is tags to web sites. I think for reference purposes it would be most useful to just bookmark the del.icio.us web site and search tags for a topic/answer to a question. Trying to create bookmarks for every single useful site for each single possible subject would just become a major amount of work, otherwise.
I must admit that I skipped creating Buttons on the toolbar as I cannot do this project on my home computer--dial-up access only at home and would take hours to do any downloading and installation. I did add a button to my blog's sidebar, though. I'll have to re-visit the assignment page for reminders and tutorials on how to use Del.icio.us.