Jon Weatherhead

AJAX Comments Refueled allows for pages to be cached without needing to invalidate on new comments, and is useful for upgrading the default comment system to not rely on page refreshes for replying to comments.
This plugin is a zero-dependency implementation. This means that it does not require jQuery.
Image by <a href="https://pixabay.com/users/geralt-9301/?utm_source=link-attribution" rel="nofollow ugc">Gerd Altmann</a> from Pixabay.
<h3>How To Use</h3>
Simply install and activate this plugin and it will work – there are no options to fuss over.

AJAX Comments Refueled

An all-AJAX drop-in for the stock comments system.

Tom J Nowell

interconnectit

Spectacula

Ever found your theme doesn’t support threaded comments? Would you like it to 
be able to have P2 style live commenting on your site? Spectacu.la Discussion 
is a plugin that replaces the commenting code in your theme with a fully Ajaxed 
comments engine.
The plugin uses memory tables for fast caching within the database (no pesky 
permission setting for you to worry about here) and adjustible polling times to 
suit a wide range of server performance.
The plugin is also developer friendly, meaning that you can easily add styles to 
your theme so that should a user implement the plugin your meticulous design can 
be carried through. By default there are two styles that should work in most 
cases. Read below for further instructions.
Once installed this plug-in will replace your theme’s comments template with its 
own fully Ajax comment template. This allows both submission and update of 
comments without a page refres letting your visitors use it more as a chat room 
than the more traditional comment system. You can also roll up replied which can 
be told to trigger at any depth you feel is best using the settings page. Also 
available on the plug-ins admin page is the option to use another or no 
stylesheet, toggle the live updating of comments and change the refresh period 
for live update.
As of version 2 you can now easily add new stylesheets to the available list by 
either copying them into the plug-in’s style folder with a comment at the top of 
the sheet that looks like this /* comment style: Stylesheet name */ or adding a 
commenting.css to your theme or child theme. Once added they will then be listed 
in the dropdown menu that shows on the admin page.
The plugin is also, of course, Multisites compatible.
=The filters=
There are a few filters available for developers to intercept the javascript, 
paramerers passed to the javascript and the CSS file location
<ol>
<li>spec_comment_css 
Passes the URL of the stylesheet through to your function to replace 
with your own file.</li>
<li>spec_comment_js 
Passes the URL to the jQuery file that controls the roll up and a few 
other elements.</li>
<li>spec_comment_local_js 
Passes in an array of localisation strings that are passed to the 
jQuery code.</li>
</ol>
To replace the CSS file you could add something like the following to your 
functions.php that would point to a comments.css file in the folder of your 
current theme. This will then override any choice made by the user on the admin 
page.
<pre><code><?php
 add_filter('spec_comment_css', 'my_css_file');

 function my_css_file() {
 return get_bloginfo('template_directory') . '/comments.css';
 }

?>
</code></pre>
<h4>Warning</h4>
Every effort has been made to make this work with as wide a variety of themes as 
possible but we can’t cover every eventuality so some themes out there will 
cause problems with this plugin without you doing something to either the 
plug-in or the theme first. The most likely cause of problems is that some of 
the CSS in the theme conflicts with the CSS in the comments. There are various 
other areas where problems could arise, such as if the theme doesn’t call the 
comments.php using the <code>comments_template();</code> template tag or if your theme 
deals with comments in an unusual way, such as placing them in a sidebar or 
calling them in using Ajax that conflicts with our own. A missing or unusual 
DOCTYPE could cause problems too, in fact there are lots of things that could 
cause strangeness. However with most of the themes I’ve tested this with it has 
worked without issue straight out of the gate and even if it doesn’t look right 
you need only disable the plug-in to go back to how things were, so nothing’s 
lost.

Spectacu.la Discussion

Spectacu.la Discussion adds threaded commenting with live AJAX comments to almost any WordPress Theme.

AJAX Comments Refueled vs Spectacu.la Discussion