bbPress Messages - User Private Messages with notifications, widgets and media with no BuddyPress needed.
GNU social based comment system for WordPress
The BTCNew WordPress plugin lets you show related conversations (from Twitter, Digg, FriendFeed & more) inline with your own comments.
Customer conversations made easy. Get you and/or your team talking to customers, beyond just email. Help desk and support simplified. Better conversat
This plugin displays the total number of threaded conversations contained within a post's comments. Coders can call the function directly.
Combine the addictiveness of online polls and the community features of comments to let your visitors rate and share opinions on famous people!
What are people saying about your post, right now?
Schmooz, The next generation of online commenting.
Wheepl widget enables blogs to have a live, unified conversation through hashtags.