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
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 â€œnofollowâ€ attributes from links in comments so search engines can follow them.
DoFollow Trackbacks - Removes â€œnofollowâ€ attributes from trackbacks
Show Top Commentators - Lists the most frequent commentators on your blog, along with links to their blogs, in your sidebar.
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 â€œwhisperâ€ 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. (via)