Disqus
The Disqus comment system replaces your WordPress comment system with your comments hosted and powered by Disqus
CommentLUV
This plugin will visit the site of the comment author while they type their comment and retrieve a selection of their last blog posts, tweets or digg submissions
Get Recent Comments
This plugin shows excerpts of the latest comments and/or trackbacks in your sidebar.
Get live comment previews on your blog.
MyAvators
MyAvatars, a plugin that shows MyBlogLog's avatars in your comments
Show Top Commenters
Encourage more discussion from your readers, by displaying their names (linked to their website if desired) and number of comments they have made recently to your sidebar.