Some additional repos to consider:
MkCommentBundle — A bundle comment system for Symfony2
Patroklo/yii2-comments — Yii2 comment system
diversen/comment — a comment system for coscms
Comment-System-Series — This is the comment system I am building on YouTube.
loudvoice-comment-system-wordpress — LoudVoice replaces the basic WordPress comments by a powerful comment system that includes logging in with 35+ social networks, spam filters and more.
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