Diigo was a pretty cool tool..I especially liked using it for articles and other reading material. Although I agree that it takes some good research knowledge in order to use this tool, juniors and seniors might be able to grasp the basics. For example, my senior zoology students are doing a research project right now and instead of printing the material from the websites, they could just highlight and bookmark them to save paper. I like that you can go back to your bookmarks later to review them. Perfect for a research project in progress.
Here are my annotated links: