Diigo is kind of del.icio.us meets local browser bookmarks meets those sticky note apps that were all over the place a few years ago. Bookmark stuff, annotate it so other Diigo users can read your notes, highlight text, post to your blog, to del.icio.us and a dozen other sites. Everything even slightly bookmarky actually – ties the desktop and the web together more profoundly than anything I’ve seen so far plus the social component should be very powerful once the installed base is big enough.
I installed the Firefox plugin this evening (there are versions for IE and Flock) and I’m already an addict. It imported all my del.icio.us bookmarks smoothly and it will mirror my Diigo bookmarks back to del.icio.us for me so I don’t have to give up on them (although without the subheading I like to add to my del.icio.us bookmarks which is annoying).