IntenseDebate

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.

Disqus is a comprehensive, worldwide blog comment hosting service for websites and online communities that leverages a networked platform. This popular service allows website owners to easily integrate a robust commenting system into their sites without the need for complex backend development. Disqus offers features such as real-time commenting, social media integration, spam filtering, and moderation tools. It also provides a single sign-on capability, allowing users to comment across multiple websites using their Disqus account. The platform's networked nature enables cross-site discussions and helps build engaged communities around content. Additionally, Disqus offers analytics tools for website owners to track engagement and understand their audience better.

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

Twikoo is a simple, safe, free comment system.