MrXHellboy

This plugin will provide you a new widget which shows the most popular posts in a unordered list. 
Widget options:Title, Amount of posts, Custom ul class, Custom li class, post type
<h3>Development Blog</h3>
<a href="http://online-source.net/category/scripts/wordpress-plugins/" title="Online-Source WordPress plugins" rel="nofollow ugc">Online-Source WordPress plugins</a>

Ron Zvagelsky

Integrates with the Disqus API to show your most popular threads (most commented posts). Can be added via sidebar widget, template tag, or shortcode.
<a href="http://presshive.com/plugins/disqus-popular-threads-widget-for-wordpress/" rel="nofollow ugc">For more information or to request additional features, please visit the plugin page</a>

Disqus Popular Threads Widget

Shows your most commented posts from Disqus via widget, shortcode, or template tag.

Popular posts widget (based on comment count) vs Disqus Popular Threads Widget