Nick Bosch

WP Dashboard Chat allows admins to communicate via a widget which displays on the Admin Dashboard. It’s perfect for quickly getting messages out to all your sites writers or having a formal meeting. Features include, @ mentions and replies, live updating, Gravatar support, auto-linking and HTML formatting. Many more coming soon!

WP Dashboard Chat

WP Dashboard Chat is a Twitter inspired widget, which displays on the admin dashboard. It allows for quick communication between users and admins.

Marko Resnik

Pikto Chat plugin is a custom chat widget that is able to display a maximum of 1000 messages at once in a window (with a scrollbar). Number of messages in the database has no limits. Gives your users opportunity to chat with each other. All currently logged in users are able to see other logged in users in chat, meaning that all logged in users chat with each other at the same time.
Features:
<ul>
<li>A button widget, which opens the chat window, and can be placed wherever you choose on the page (in widget areas).</li>
<li>You can choose the number of messages you would like users to have in certain conversation history: 5, 50, 100, 200, 400, 1000 </li>
<li>The color scheme of the chat window (background and text color) can be also changed from the widget admin side just like the number of messages</li>
<li>Chat window is easily closable by clicking the “X” sign in the upper right corner of the chat window, or by clicking the chat button in the widget area of the page</li>
</ul>

Pikto Chat

Pikto Chat plugin is a custom chat widget that is able to display a maximum of 1000 messages at once in a window (with a scrollbar).

WP Dashboard Chat vs Pikto Chat