The Top Commentator widget – written by Webgrrrl – does pretty much what the name suggests.
It displays a list of the top contributors to your blog discussions, ranked by total comment count, along with their gravatar and a hyperlink back to their own URL.
You can place the list in any widgetized area of your WordPress site.
All settings can be configured in the widget control panel, such as how many commenter names are displayed, and how often the list refreshes. You can also filter specified email addresses and URLs to keep undesirables off the list.