This add-on enables nested commenting (comment on comment, reply to comment) for ExpressionEngine 2.x.
You can have unlimited depth of comment nesting.
Threaded comments module is using same database table that first-party comment module does will respect all commentings settings you have. It contains all extension hooks that comments module does, so all of your extensions will work.
All variables available with exp:comment:entries are available with exp:threaded_comments:display as well.
You can use different html for different levels of comments. Commenters are able to subscribe to notifications only for selected thread.