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

BuddyDev

Brajesh Singh

This plugin is deprecated. It is not compatible with WordPress 4.0+ . Please do not use it. We won’t be able to support it in future. 
Multisite Global Terms allows all sites across a WordPress multisite network to share the categories/tags/terms.
It uses the main site’s table to store all the terms. 
Please visit the blog post on <a href="http://buddydev.com/wordpress-multisite/want-global-categories-tags-taxonomies-across-wordpress-multisite-network/" title="Multisite Global Terms" rel="nofollow ugc">BuddyDev</a> for support or to ask a questions.

Multisite Global Terms

Thanks! Didn't install, but used your code!!! 

Nice!

I didn't install this plugin, but I'm using your code on a new multisite build. Thank you for writing this plugin as it was exactly what I was looking for.

-Steven

This is exactly what I was looking for!

Allow global terms across all blogs on a multisite installation.

Sitewide (Admin) Header/Footer vs Multisite Global Terms