In metadata, Annotea was designed as an RDF standard[1] sponsored by the W3C to enhance document-based collaboration via shared document metadata based on tags, bookmarks, and other annotations.
Users that access any web documents can also load the metadata associated with it from a selected annotation server (or groups of servers) and see a peer group's comments on the document.
Similarly shared metadata tags can be attached to web documents to help in future retrieval.
Annotea is part of the W3C Semantic Web efforts.
Other projects consists of Plugins for Firefox/Mozilla or Annotatio Client which interacts with most browsers per JavaScript.