Two long years ago, I wrote an article here titled “Document Collaboration (with Comments!).” As a long-time blogger who assists others with their articles, I’ve experimented with various tools to facilitate this process. I included some options in that post, but none are perfect. The well-known options come with drawbacks, and the lesser-known ones come with risks (one is already defunct).
It was noteworthy that Matt Web managed to code up a solution over a weekend: mist.