For those of you unfamiliar with Diigo, it is a social bookmarking tool that allows you to annotate, archive and organize websites. With Diigo, you can save websites into a public or private library, highlight information within text and images, add sticky note comments directly on websites, tag sites for easy retrieval later, and share bookmarks with friends, groups, and networks. Diigo integrates the use of tags and folders, highlighting and clipping, sticky notes and group-based collaboration to better manage online information.
http://blog.learningtoday.com/blog/bid/26051/Exploratory-and-Collaborative-Learning-with-Diigo