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utterances is an innovative, lightweight comments widget that leverages GitHub issues as its backend. This open-source solution is particularly popular among developers and technical bloggers. utterances works by creating a GitHub issue for each page or post that requires comments. When users leave comments, they are actually creating comments on the corresponding GitHub issue. This approach offers several advantages, including version control for comments, markdown support, and the ability to leverage GitHub's user authentication system. utterances is easy to set up, requires no database, and is completely free to use. Its minimalist design and GitHub integration make it an excellent choice for developer-focused websites and blogs.

Livefyre is a platform that integrates with the social web to boost social interaction.

IntenseDebate is a blog commenting system that supports Typepad, Blogger and Wordpress blogs. The system allows blog owners to track and moderate comments from one place with features like threading, comment analytics, user reputation, and comment aggregation.

Twikoo is a simple, safe, free comment system.

Question2Answer (Q2A) is a popular open-source Q&A platform for PHP/MySQL.