Diigo is a social bookmarking tool that allows students to create online bookmarks, share them with others, and even annotate, highlight and leave sticky notes to be read privately, shared with a group, or displayed publicly. You can download the diigo toolbar to use regularly or use the diigolet without any installation necessary.
A brief intro. to the program can be seen by watching this tutorial.

Diigo V4: Research ~ annotate, archive, organize from diigobuzz on Vimeo.