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
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jimmysun

Simple Popular Posts is a plugin that creates a widget for you to put on your sidebar. The widget shows the most popular posts on your blog, based on the number of comments.
It is a very simple and basic plugin. It only has 3 options: 
– the widget title 
– the number of posts you want to show 
– whether you want to show the number of comments or not
Based on the Most Popular Posts plugin by Wes Goodhoofd (http://www.wesg.ca/2008/08/wordpress-widget-most-popular/)

Simple Popular Posts

Creates a very simple and basic widget for your sidebar to display most popular posts on your blog based on the number of comments only.

Popular posts widget (based on comment count) vs Simple Popular Posts