The latest version 5 of Google Toolbar integrated Google Notebook, the service that allows you to save interesting parts from a web page for later. Google also started to highlight the fragments from a web page that were previously saved and show your comments in a tooltip.
Google, Google Toolbar, Google Notebook, Annotation
You can't export the annotations or subscribe to someone's public annotations, but it's not hard to see these features added in the next iterations of Google Toolbar. Trailfire already lets you find trails, "collections of web pages, assembled and annotated by any Trailfire member". The collections of notes could be used to guide people to the most important parts of a Google search result or they could become a part of a dynamically-built encyclopedia page.
The current implementation from Google Toolbar provides you with a simple way to share clips from web pages with your friends and also add comments.