There are a plethora of silo oriented web-based "sticky note" systems, such as Google's Sidewiki, and stand-alone multimedia annotation systems. The primary complaint about these systems is that the annotation is locked in to the web site or tool which was used to create them, and cannot be seen or managed with any of the other systems.
Scholars wanting to annotate have to learn different annotation clients for different content repositories, have no easy way to integrate annotations made on different systems or created by colleagues using other tools, and are often limited to simplistic and constrained models of annotation.
Open Annotation goals:
Data Model Guide for Open Annotation Systems: The Open Annotation Data Model specifies an approach for associating annotations with resources, using a methodology conformant with the Architecture of the World Wide Web and the Linked Data initiative. It draws on the Annotea model, as well as more recent extensions of that model.
Open Annotation Data Model - Alpha 2 version: http://www.openannotation.org/alpha2/
Open Annotation Data Model - Latest version: http://www.openannotation.org
7 sept 2010: this standard is currently under development. It is interesting to be aware of the existence of these developments