I like the fact that with del.icio.us you can access your bookmarks from any computer. I often feel a bit lost when I am on a different machine and don't have access to all my favourites. I also like the fact that you can tag these bookmarks. In a previous job, I created a directory of useful web resources and I wish I had been aware of del.icio.us then, as it would have been better than trying to create a linear list. This is the purpose for which most libraries seem to have used del.icio.us. I particularly like the tag cloud option, both as a visual statement but also as the same information appears less overwhelming. I found that the University of Dundee allowed you to bookmark catalogue entries with del.icio.us.
I am a bit behind so I have not explored diigo but I have come across a library using it in previous work. It was for exactly the same purpose as the others I looked at use del.icio.ius.
Also, unfortunately I have not had a chance to make use of Evernote but I have every intention to. This is because I currently have a notebook with information all over the place and I too sometimes struggle to read my own writing! Being able to access my notes anywhere and not having to be attached to my notebook will be advantages as well.