I have been using Diigo for a few years now and the feature I like best is the ability to belong and share links with groups. I keep up-to-date with the help of my virtual colleagues! I have not used the highlighting and notes features yet, but plan to do so. The short video embedded below shows the new features of Diigo 5. Check out my Diigo library here.
Diigo V5: Collect and Highlight, Then Remember! from diigobuzz on Vimeo.