On wordpress, threaded comments have not been a big issue so far, but with code tweaks and DB updates required, most of the plugins that “promised” a threaded reply comments were not popular if not with some hardcore Wordpress coders and the like.

Well, to end this situation, we have a brand new wordpress plugin that let’s you reply to comments on non-threaded comment templates by adding a simple “@” sign just like in Twitter.
With the new plugin installed, readers can use the @ symbol to reply to any comments above them and it would show a original comment preview near it and gives you a threaded effect.

Technically :-
This plugin allows you to add Twitter-like @reply links to comments,
including a preview of the comment replied to.
When clicked, those links insert the author name and a link to the
comment you are replying to in the textarea, and adds a Preview of the
original comment to the @Reply (on hover using javascript).

Download the plugin here. Plugin homepage here.