Maxaud

(NOTE: Before saying this plugin is broken, please understand what this plugin does. It’s meant for WordPress installs that have multiple NETWORKS.)
Adds the ability to have different themes available to different NETWORKS in a multisite atmoshpere.
WordPress allows for multiple “networks”, these are not to be confused with sites. A network holds blogs underneath of it.
This plugin will allow you to separate themes into different directories so they’re only accessable and viewable to the networks you choose.
The plugin will attempt to create a directory in the /wp-content/ directory named /ms-themes/. If it fails to create it, then you will need to make one.
In the /ms-themes/ directory you will create a folder for each network that you want to allow only certain themes to.
Example: 
If you have a theme named “Snarfer Theme”, and you want to only allow it to be viewable/available to the network (and all sites on that network) with the site_id of 7, you would place the snarfer theme directory in /wp-content/ms-themes/7/
Future releases:
<ul>
<li>Network option to turn off auto creation on/off network directories, index.php files.</li>
<li>Network option to display network ID in the footer of sites so network admins can easily tell what network a site is under.</li>
</ul>

Multisite Themes

Adds the ability to have different themes available to different NETWORKS in a multisite atmoshpere.

jroakes

This plugin will give you the ability to pull up any (non-Super Admin) user and in one click add that user(with a global role) to all sites in your MU install. Likewise, if you want to remove a user from all sites it is one click as well. CAUTION: WordPress has an optional mechanism when deleting a user from a site that will allow you to assign that users posts to another user. I have ignored this for this release. Please take a DB backups until you are used to this plugin and Multi-Site.

Multi-Site User Replicator 3000

WP3.1 multisite "mu-plugin" that allows you to add a user to all sites or no sites. Just drop in mu-plugins.

Multisite Themes vs Multi-Site User Replicator 3000