Christian Foellmann

Jason LeMahieu (MadtownLems)

<blockquote>
The plugin is deprecated and reaches a hard end-of-life date on 31. January 2018!
</blockquote>
<h4>Alternative</h4>
You should switch to <a href="https://wordpress.org/plugins/multisite-enhancements/" rel="ugc">Multisite Enhancements</a>. It provides the same functionality and is actively maintained.
This plugin gives you a count and the listing of sites using your installed plugins.
Usage data is cached in a Transient (non-autoloading) but the data collection process can be a very expensive operation depending on plugin and (especially) site count. 
Check the <a href="https://wordpress.org/plugins/wpmu-plugin-stats/faq/" title="Frequently Asked Questions" rel="ugc">FAQ</a> for more details on caching.
<ul>
<li>Requires a WordPress Multisite Installation</li>
<li>JavaScript is required to toggle the list of sites using a plugin</li>
</ul>
<h4>Development</h4>
<blockquote>
The plugin has reached its end-of-life. No updates and/or support after 2018-01-31
</blockquote>
<ul>
<li>GitHub Repository: <a href="https://github.com/wp-repository/wpmu-plugin-stats" rel="nofollow ugc">wpmu-plugin-stats</a></li>
</ul>

WPMU Plugin Stats

Gives network admins an easy way to see what plugins are actively used on the sites of a multisite installation

DeannaS

This plugin creates site-wide topics. Each blog can be identified as belonging to a single topic. Blog owners can select a topic for their blog at creation time, and through a menu under settings. Blog owners can also choose whether or not to include their content in any site-wide aggregated content via the Blog Topics Settings menu.
Site Admins can set up the site-wide topics, select a “featured topic,” and manage blog’s topic settings through the Site Admin -> Blogs -> Edit menu.
This plugin comes with 5 optional widgets (in the widgets subdirectory).
<ol>
<li>BT – Topic Name – displays the name of the topic of the current blog.</li>
<li>BT – Related Blogs – displays a linked list of other blogs in the same topic.</li>
<li>BT – Related Posts – displays the title of the N most recent posts in the same topic as the current blog.</li>
<li>BT – Featured Topic w/Posts – displays the N most recent posts from the “featured” topic.</li>
<li>BT – Topics w/Posts – displays the N most recent posts from each topic, with the option of excluding selected topics.</li>
</ol>
Some of the widgets rely on theme code for topic listing pages and site listing pages, referred to as “portal links.” All portal links can be turned on and off in the widgets. This plugin comes with a sample theme to help you learn how to modify your own theme to incorporate the portal elements.

Blog Topics For WPMU

Allows users to categorize blogs by topic. Allows users to categorize blogs by topic. Includes multiple optional widgets and optional sample theme cod …

WPMU Plugin Stats vs Blog Topics For WPMU