Spam Karma – Analyzes comments for spam based on a karma system.
Bad Behavior – Prevents spambots from even accessing your site by analyzing their HTTP requests.
Did You Pass Math? – Asks commenters a simple math question before their comments are posted.
Comment Timeout – Closes comments on old posts.
BlogFollow – Shows a snippet of the commenter’s blog at the bottom of the comment.
DoFollow – Removes “nofollow” attributes from links in comments so search engines can follow them.
Nicer Trackbacks – Improves the appearance of trackbacks from other blogs.
WP-NoSh*t – Filters bad words from comments.
Numbered Comments Adds numbers to comments.
AuthorHighlight – Highlights comments by the blog’s author.
Comment Highlighter – Styles comments based on the author’s name, email, or URL.
wp-chunk – Truncates long URLs in comments to prevent them from stretching the page.
Enable rich comments
Custom Smileys – Adds clickable smiley emoticons to comments.
TinyMCEComments – Turns the comment box into a full-featured rich text editor.
Live Comment Preview – Allows readers to preview their comment before posting.
Edit Comments XT – Lets commentators edit their own comments.
Gravatar – Displays user-selected avatars from a central database next to comments.
Favatars – Pulls the favicon from a commenter’s blog or website, and uses that for their avatar.
MBLA – Adds user images from MyBlogLog to comments.
OpenID Comments – Lets readers comment with their OpenID identity.
Show off comments
WP-Most Commented Posts – Displays posts with the most comments in the sidebar.
Latest Comments with Avatars Reloaded – Shows the latest comments, along with the MyBlogLog avatars of the commenters.
Allow better conversation
Simple CoComments – Tracks the conversation across blogs.
Comment Karma – Digg-style voting on comments.
Quoter – Allows users to quote a previous comment in their comment.
Whisper – Lets users “whisper” comments that are only visible to one person.
Moderate comments effectively
Comment Twitter SMS Notification – Notifies you of new comments through Twitter SMS.
Bannage – Adds the ability to ban users from commenting by name, email, or IP.
CommentCentral – Allows batch opening and closing of comments on multiple posts.