I have a personal del.icio.us site. I have a professional one for work. My personal one is such a mess that I would like to start over. It is really hard to organize bookmarks well without simply having lists of lists. I am not sure that this idea of tagging works particularly well to sift out unnecessary stuff either. One has to be decent at tagging for it to really be effective. and perhaps even to have designed a system beforehand. I should look at other folks' sites to see how effective their set-up is.
For me, I initially created my personal account so I could save bookmarks from several places... home, work, work computers #1, #2, and #3, etc... Since then, two-ish years ago, I haven't really used it again. In my professional account, I have kept things pretty simple. There are only links to pathfinders I have made and a couple sites that I just found through the SLL site. I am working on how to keep it organized. I really think it takes a lot of practice to get good at the organization and until I get good , the KISS (keep it simple stupid) solution is the best.
Again, a plea... What are you doing with del.icio.us in your school library? How do you use it with teachers and students? Also, in the list of links to Discovery Resources, it would be helpful to see some links to school library del.icio.us sites! I linked mine above.