Wordpress Plugins
Front End PM Private Messaging for WordPress
Here is another WordPress plugin that adds a private messaging system to your website. Front End PM as the name suggests handles the messaging through the front-end of your website. This is especially helpful if you want to keep your users out of your dashboard.
This plugin can handle threaded and individual messages. It also has desktop notifications. Admins can set max amount of messages per user and send public messages. They can also apply custom CSS and time delays between messages. The pro version of this plugin has group messaging, read receipt, and other advanced functions.