Disqus allows posting as a guest under their free plan. It’s also important to know and understand the difference between guest commenting and anonymous commenting. Disqus allows guest commenting, without having to sign up or creating a Disqus profile. A person commenting as a guest, will not be subscribed to email notifications of new comments or any replies on their comments. Also, guest comments don’t get instantly posted and must be approved by a site moderator to appear on the page. Anonymous commenting can be done by a registered user with a pseudonym as well.
than Disqus, IntenseDebate also allows guest posting and it’s completely free
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