I have been using Del.icio.us for over a year now.  I have used as a Quick Tip a couple of times and also put that information on the WALT Blog.

It is great to find sites that others have already found on a topic.

I think it could be a great tool for teachers and/or for use to help students when teachers give an assignment.  A tag for the topic and/or teacher or school could be used to group tools for students.