Textensor, formed from a group of researchers in Edinburgh working on tools for authoring structured content, have a beta of a new tool for annotating online documents: Notate. I've only tried it for a day or so, but it's impressively built and very friendly to use. Any web page can be cached on the textensor server, then annotated by a group who can see each other's notes. Similar to Marginalia, this could be useful for online, collaborative peer review.
Click the logo in the top right of this page to try Notate (it fetches javascript from their server, which isn't ideal, but for some reason it doesn't work if the script is hosted locally).