Wikipedia
Hypothes.is
Hypothes.is is a 501(c) open-source software project that aims to collect comments about statements made in any web-accessible content, and filter and rank those comments to assess each statement's credibility. It has been summarized as "a peer review layer for the entire Internet."
As of March 2016, Hypothes.is is the 447,510th most popular site on the Internet according to Alexa.com.