I heard, "If you like Delicious, you'll love Diigo! It's Delicious and SO MUCH MORE." So I had to check it out.
The "SO MUCH MORE" entails the ability to highlight and annotate websites and share your annotations with others.
For my personal use, I'm not a big fan of other people's comments scattered about on sticky notes attached to the pages that I visit.
For student use, I can see some potential here. We English teachers encourage our students to interact with the text, and Diigo makes that possible.