Diigo, a Social Annotation tool

"The name "Diigo" is an abbreviation for "Digest of Internet Information, Groups and Other stuff."

Diigo (dee'go) is about "Social Annotation". By combining social bookmarking, clippings, in situ annotation, tagging, full-text search, easy sharing and interactions, Diigo offers a powerful personal tool and a rich social platform for knowledge users, and in the process, turns the entire web into a writable, participatory and interactive media.

The social annotation service introduced by Diigo allows users to add highlights and sticky notes, in situ, on any web page they read. Imagine a giant transparency overlaying on top of all the web pages. Users can write on the transparency as they wish, as private notes or public comments. And they can read public comments on the transparency left by other readers of the same page, and hear their "two cents" and interact with them."

http://www.diigo.com/