Diigo

Diigo is a social annotation service enabling users to add highlights and notes to any web page.

An abbreviation for the 'Digest of Internet Information, Groups and Other stuff,' Diigo is a social annotation service that lets users add highlights and notes _in situ_ to any web page. These overlayed comments made through Diigo can be made either public or private; other Diigo users can then read public comments and reply/interact to comments. Diigo combines aspects of social bookmarking, full-text search, annotations, tagging, and sharing. A browser plugin allows you to share your annotations directly with others, and you can save annotated pages to numerous other bookmarking services as well.