A commenting server similar to Disqus and IntenseDebate. CURRENTLY UNMAINTAINED
Similar to acts_as_commentable; however, utilizes awesome_nested_set to provide threaded comments
Plugin/gem that provides comment functionality
Integrates Disqus into your Ruby-powered site. Works with any Ruby website, and has view helpers for Rails and Merb.
A Rails engine for comments.
Rails: Make an ActiveRecord resource ratable/reviewable (rate + text), without the usual extra code-smell.
Opinio is an engine used to add comments functionallity to rails 3 applications.