tempranova

This is a simple plugin that uses AJAX to load comments, converting the “Newer/Older Comments” button into a button that loads more comments instantly.
This plugin also paginates nested replies. You can customize styling in whatever way you choose.
Using Settings > Comment Load More, user can set desired pagination (currently both comments and replies have the same number).

Comment Load More

This plugin allows you to load more comments instantly via a button push (AJAX), instead of pagination that reloads the page.

mg12

AJAX paging plugin for comment pages in WordPress 2.7 or higher versions.
为 WordPress 2.7 的评论分页功能增加 AJAX 处理.
Features:
<ul>
<li>AJAX comment paging</li>
<li>Without any JavaScript framework</li>
</ul>
Demo:
http://www.neoease.com/themes/
Supported Languages:
<ul>
<li>US English/en_US (default)</li>
<li>简体中文/zh_CN (translate by <a href="http://www.neoease.com/" rel="nofollow ugc">mg12</a>)</li>
</ul>

AJAX Comment Pager

AJAX paging plugin for comment pages in WordPress 2.7 or higher versions.

Comment Load More vs AJAX Comment Pager