The top commentators plugin is available to download here and here’s some more information on it:

The Top Commentators Widget a sidebar widget to show the top commentators in your WP site. Adapted from Show Top Commentators plugin at Personal Financial Advice, this widget is easier to manage via the control form (no need to edit the PHP file); additional options are also available to make it more flexible. Read the FAQ section on how to customize the widget, and take a look at the screen shot to see the control form for customizing the Top Commentators Widget.

Prerequisites: Your blog theme needs to support widgets.

Installation:
1. Download the top commentator plugin from the location above.
2. Extract the files on your machine.
3. Using your hosting account file manager or by FTP, upload the ‘top-commentators-widget’ folder to your blog’s plugin folder:
/wwwroot/yourblogname.com/wp-content/plugins for Windows hosting
or
/webspace/httpdocs/wp-content/plugins for Linux hosting
4. Login to your WordPress admin panel: yourblogname.com/wp-admin
5. Activate the top commentator plugin using the admin panel. We suggest backing up your site before doing so, just incase there’s any problems.
6. Browse to Appearance >> Widgets in your WP admin panel and you should see that your top commentator widget has appeared there.
7. Drag the Widget on to your sidebar and you’re done installing it.

That was simple, wasn’t it? You’ve installed the Top Commentators plugin and now you need to configure it. Check out tomorrow’s post on how to do this.