Diigo is fun! It is my absolute favorite. This service has an enormous amount of tools and features — it is more than a bookmarking service, it is a complete tool for online research.
To start with the basics, Diigo is easy to use. Tabs and pull-down menus in addition to the tag cloud makes it easy to navigate, sort and edit bookmarks and tags. I have 1000+ bookmarks, so this is important to me. If you do research online, you’ll soon value these tools.
In addition to being a bookmark tool, Diigo lets you add sticky notes and highlight text on any web page just like you would on paper. This way, information can be highlighted on web pages for easy scanning later and snippets of information can be saved for sharing, editing and reference. The highlights and sticky notes are private by default, but can be shared with your Diigo friends in a number of ways.
The possibilities for social networking on Diigo are unequaled in the world of social bookmarking. Diigo has a group tool for collaborative research that allows groups of people to pool their findings through shared bookmarks, highlights, sticky notes, and forum discussions. There are also communities formed around web sites (like Wikipedia) and tags (like Web 2.0).
To learn more, have a look at this 4 minute introductory video.