Is there an open crowd-based peer-review tool?
Besides http://peerevaluation.org, there is http://www.peerageofscience.org; and there is also reddit.com. Reddit is most like SE in terms of user interaction, however they have some problems with quality control. It is "open" in the sense that anyone can offer material and make a comment (review): it is "not entirely open" in the sense that authors and reviewers have the option of using a pseudonym, so real identities are usually not known. The other two sites are open in the latter sense, that authors and reviewers are openly identified (as far as I can tell), but peerageofscience requires reviewers to "have published a peer reviewed scientific article in an established international journal, as first or corresponding author", so it is closed to reviewers without that credential. (I am not sure about peerevaluation but it does not seem to require any qualifications from reviewers though it does ask you to categorize yourself).