Disqus offers the best add-on tools for site owners to power discussions, increase engagement, and earn revenue.
Commento is a popular, fast, bloat-free commenting system that you can embed on your blog, news articles, and more. Open source at its core, Commento guarantees you privacy and does not include third-party trackers and advertisements, unlike alternatives such as Disqus.
No updates since Aug 2019 & won't work with the latest PHP versions.
An embeddable comment widget for websites, blogs, ecommerce & static pages. Joint Comments distributes JOINT tokens (erc-20 based cryptocurrency) to publishers based on the value of traffic.
It is free to use up to 250k page views / month.
coComment is a service for managing, powering and researching conversations online. When using coComment, you can keep track of your comments across any site, share them with friends, and get notified when you get a response.Discontinued
Seems to be discontinued as of April 2 2013.