Following is a list of comment related WordPress plugins that are highly rated (at least 4 stars ) and are compatible with WordPress version 3.0 or higher
Help Encourage Commenting
Top Commentators Widget – Adds a sidebar widget to show the top commentators in your WP site.
Subscribe to Double-Opt-In Comments – Allows readers to receive notifications of new comments that are posted to an entry, with double-opt-in Feature. First, the user will get an e-mail with a confirmation link, after the user has confirmed the subscription, he or she will be noticed about new comments.
Improve comment appearance
Greg’s Threaded Comment Numbering – Adds numbers to comments.
Easy Gravatars – Add Gravatars to your comments without modifying any template files. Just activate, and you’re done!
Fight Comment Spam
Akismet – Antispam plugin from the creators of WordPress that uses a central database of spam comments to flag spam. Requires a free API key from WordPress.com
Bad Behavior – Prevents spambots from even accessing your site by analyzing their HTTP requests.
Comment E-Mail Verification – Forces commentators to use a real e-mail address when commenting. The process is simple: For first time commenter, the comment is kept in moderation automatically. The commenter gets a confirmation e-mail with a link in it. Clicking that link takes the comment out of moderation and approves it.