Besides simply tagging and storing bookmarks online, Diigo seems to offer a couple of unique features focused on the idea of annotation and collaboration.
Diigo's power comes from the Diigo toolbar. With the toolbar, a user can quickly access stored bookmarks tagged as "Links" (or other tags by simply changing the filter) via a traditional folder. From this toolbar, searches can be executed across all bookmarks, blog search engines, or any incredible number of sources of interests such as Movie/TV or Shopping. There is also a one-click access to your "Inbox" on the toolbar - essentially allowing real-time access to tags you are trying to track.