The Web Annotation Working Group has published a Working Draft of Web Annotation Protocol. Annotations are typically used to convey information about a resource or associations between resources. Simple examples include a comment or tag on a single web page or image, or a blog post about a news article.
From Section 1.1 of the Draft
The primary aim of the Web Annotation Protocol is to provide a standard set of interactions that, when implemented, will allow annotation clients and servers to interoperate seamlessly. By being able to discover annotation protocol end-points and how to interact with them, clients can be configured either automatically or by the user to store annotations in any compatible remote system, rather than being locked in to a single client and server product pair.