If you have ever enjoyed using a social bookmarking site like Delicious.com, you may really like Diigo (pronounced "Deego").
After attending the NECC 2008 Conference this year, Diigo seemed to be what all the educators were talking about.
I haven't fully explored it, but it seems to have a lot more capabilities. You can highlight and sticky-note web text. You can have group discussions, notify and share bookmarks, and it seems to be the hottest thing. Check it out!