Check lists at
  https://news.ycombinator.com/item?id=24676152

Vssue (github, bitbucket etc. login required)

https://iwantsourcecodes.com/open-source-comment-system/ PHP
https://github.com/tessalt/echo-chamber-js 3766★
https://github.com/aroc/side-comments
https://github.com/arunoda/open-comment-box ? Meteor?
https://discuss.flarum.org/ ?
https://code.google.com/archive/p/comment-it/
https://github.com/pvorb/tala
https://github.com/pvorb/node-opinions
https://github.com/scotch-io/laravel-angular-comment-app
https://github.com/kenota/kommentator
https://github.com/alexandru/OldBuzzEngine (6 years old, 54 stars)
https://github.com/CMGS/ymir
https://github.com/inaimathi/clomments
https://github.com/hhandoko/diskusi
https://github.com/rochacbruno-archive/flasqus
https://github.com/Purexo/DeportedCommentary (python, French)
https://github.com/theikkila/comments
https://github.com/alejandro/node-comm
https://github.com/joway/Chirp
http://ecomment.su/ (Russian; https://bitbucket.org/thorn-workshop/ecomment.git doesn't work,
  download from http://www.php-s.ru/comments/eComment/)
https://github.com/c4all/c4all
https://github.com/ouija-io/ouija (138 stars; last update 2014. For Ghost.
  Backend is Goinstant which died, so ouija)
 

Ruby on Rails apps:
  https://github.com/the-trash/the_comments (315 stars)
  https://github.com/Draiken/opinio (125 stars)
  https://github.com/lml/commontator (185 stars)