The Private Messages For WordPress allows users of WordPress blog send private messages (PM) to each other, just like in a forum.
User Messages will allow the users registered on your blog to have a private Inbox on your site.
bbPress Messages - User Private Messages with notifications, widgets and media with no BuddyPress needed.
Enhanced Private Messages System for BuddyPress
Admin User Messages is a simple plugin that allows communication just between Admin and single users, but not a communication between user and user.
Easily add Private Messaging for your users to your WordPress site with Cartpauj PM (Private Messages).