Amzil Ayoub

Real-time user status indicators for WordPress admin. Displays 🟢 (online) or 🔴 (offline) badges next to users, automatically updating every 30 seconds. Optimized for high-performance on large sites.
<h3>Features</h3>
<ul>
<li>Visual status indicators (green/red circles)</li>
<li>Tracks all user activity (frontend and admin)</li>
<li>Auto-updates without page reloads</li>
<li>Lightweight (single optimized SQL query)</li>
<li>Works with 30,000+ users</li>
<li>Zero configuration needed</li>
</ul>

Session Monitor

This plugin adds real-time user status indicators to the WordPress admin users list. It  tracks activity with minimal impact, even on large sites.

bartschatten

This plugin will display the online/offline status of defined server in your sidebar. 
This idea was born in case of setting up dedicated trackmania servers and 
I want to show the actually stats for the blog visitors / clan members. 
The plugin is easy to configure and easy to use. Now with Trackmania colorcode support. 
See installation section for more information.
Check out http://www.martin-fredrich.de/wordpress/wordpress-plugin-wp-srvstatus/ for an detailed description.
Translations
Not available at the moment, english only.

WP Server Status

This plugin will display the online/offline status of defined server in your sidebar.

Session Monitor vs WP Server Status