Philipp15b

This plugin allows you to add your code/text to the headers and footers on all the blogs in the blog network and in the backend of all blogs too.
In addition, it is possible to replace the footer bar text in the backend completely.
You can access the settings under Network Admin -> Settings.

Sitewide (Admin) Header/Footer

Change the header/footer of the front- and backend of WordPress Multisite

Adadaa

This plugin creates four (4) seperate RSS 2.0 feeds from posts, comments, pages, and one combined [posts & pages] across all blogs on your site.
Works both on WPMS upto 4.9.20
This will skip the first comment and page of a blog; also will not include private, spam, mature and deleted blogs.
Both Subdirectory and VHOST installations are now supported. Thanks to DaryL.
Features:
<ul>
<li>Generates sitewide combined [posts & pages] feed</li>
<li>Generates sitewide posts feed</li>
<li>Generates sitewide comments feed</li>
<li>Generates sitewide pages feed</li>
<li>Client-side caching via ETag headers</li>
<li>Admin screen to configure options</li>
<li>Compliance with xhtml and xml standards & many more</li>
</ul>
For more explanation and to leave any suggestions to improve or bugs, please visit on my blog : <a href="http://1uthavi.adadaa.com/ada-wpmu-sitewide-feed-plugin/" rel="nofollow ugc">Ada WPMS Sitewide Feed</a>
<h3>Usage</h3>
Compatible with WP Multi-Network plugin . You can have different blog ID than 1 to trigger the full feed.
<h3>To Do</h3>
<ul>
<li>Comply fully for language translation</li>
<li>Exclude blogs by ID</li>
<li>Enable/disable individual sitewide feeds</li>
<li>Option to include the blog title in the post title shown in the feed</li>
<li>Consider changing the function ada_get_avatar_url() to use the core hook get_avatar(). So it can work with WPMUDEV’s Avatars for Multisite.</li>
<li>Option to exclude by post types</li>
</ul>

Ada WPMS Sitewide Feed

Creates four rss 2.0 feeds showing recent posts, comments, pages, and one combined [posts and pages] from all blogs.  Works both on WPMS, and WPMU

Sitewide (Admin) Header/Footer vs Ada WPMS Sitewide Feed