Comment Gatekeeper

The Machine Learning Antispam Plugin uses Machine Learning to detect spam and adult content comments and mark them as spam.

Restrict user to view the unapproved comments in dashboard.

Adds a template tag to allow you to display only the trackbacks for your posts/pages.

eCommerce Disqus Comments and Ratings gives you better control over palcement

Twitter Comments system. Automatically retrieve Twitter Replies and/or Twitter Retweets to your Blog's Posts using WP built-in comment system.

You can change comment settings post by post on WP 2.7 or greater.