This is the the Annozilla project, designed to view and create annotations associated with a web page, as defined by the W3C Annotea project. The idea is to store annotations as RDF on a server, using XPointer to identify the region of the document being annotated.
J. Kahan, and M. Koivunen. WWW '01: Proceedings of the 10th international conference on World Wide Web, page 623--632. New York, NY, USA, ACM Press, (2001)