Diigo is a content sharing and bookmarking tool. It pulls double duty as a way to clip things from the Web, as well as share them with others. What makes the service really interesting is that you can use it to grab entire Web pages, and make annotations right on top of the content. Then you can share it with others, or view it even when the source site is no longer there. It's like a personal archive.
Along with the personal nature of Diigo, the service also has groups that users can join to participate in collaborative research. Here you can add some of your bookmarks to a group collective and be notified when there new items. The service also tracks some of the most popular saved items and puts them on a leaderboard, which can be a great way to find new and interesting Web content.