Interesting, I also just got spam from Third Voice about the latest version of their still-IE-only browser plugin. (I won't sell my soul for that...) Annotator predates Third Voice for public commentary on web pages hosted elsewhere, and itself owes a lot to the CritLink mediator. It's server based, so much more portable than Third Voice -- once you've navigated the frames, it even looks decent in lynx. Right now, however, it's very slow, mostly at the stage of very inefficient pattern-matching to place notes in the text. It's just a set of Perl scripts talking to a mySQL database if anyone wants to poke around :-).
Ultimately, I'd want to use tools like this in the Openlaw project for collaborative development and editing of documents. Mailing lists have their limits. Law firms must be among the most technologically backward places, so I can claim this to have some sort of relevance to what I'm supposed to be spending my time doing.