From the about page (archived copy):
Digress.it is a WordPress plugin that offers paragraph-level commenting in the margins of a text. Digress.it is geared toward in-depth discussions of longer documents: article, essay or even book-length.
Blogs aren’t bad for having conversations, but comments tend to get unwieldy, and can feel unmoored when the original post is long. To solve this, Digress.it lets you run blog-style comment threads — digressions, if you will — off of individual paragraphs. To do this efficiently, we’ve re-jiggered the conventional post-discussion hierarchy of blogs, moving the comment area from beneath the post to beside it (floating to the right) — hearkening back to the age-old practice of scribbling in page margins. We see great possibilities for educators, literary groups, political or civic activists, legal scholars, and pretty much anyone who wants to do a communal close reading.
Digress.it has its roots at the Institute for the Future of the Book with a project called CommentPress [now renamed CommentPress Core; ParticipateDB entry]. Digress.it represents a re-envisioning of the project with an emphasis on creating a public space where people can publish documents and encourage thoughtful discussions.
Digress.it is open-source and available on "Github"https://github.com/eddietejeda/digressit.
Digress.it is developed by Eddie Tejeda, Ben Vershbow, Joss Winn, Alex Bilbie, and Jesse Wilbur.
Update: digress.it has been discontinued. In order to access the site, view an archived copy.