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.
The Spoon Browser Sandbox lets you run all popular web browsers, including Internet Explorer, Firefox, Chrome, Safari, and Opera, directly from the web, with no installs.