Roy Tanck

This plugin will help you:
<ol>
<li>Find plugins and themes that are not active on any blog.</li>
<li>Determine the impact in case a theme/plugin needs to be upgraded, removed or reconfigured.</li>
<li>See if a widget is in use on any blogs, and track down where.</li>
<li>Find users who do not have a role on any site (and are not network administrators).</li>
</ol>
This plugin currently does nothing on single-site WordPress installs.

Network Admin Assistant

Helps you manage a WordPress network by displaying useful statistics.

feedmeastraycat

WordPress removes the Update Services ability (Settings – Writing) when you create a network (aka enables WordPress MU/Multisite). 
Activate this plugin to get it back.
Note: This is only for WordPress setups with a network, aka multiple sites, aka multisites, 
aka WPMU in WordPress 3 and later.
This plugin might work with older WordPress MU sites. I haven’t tested it though. If you do, please 
let me know how it turned out.
<h3>Files</h3>
<ul>
<li>/activate-update-services/activate-update-services.php</li>
<li>/activate-update-services/README.txt</li>
</ul>

Activate Update Services

WordPress removes the Update Services ability when you create a network. Activate this plugin to get it back.

Network Admin Assistant vs Activate Update Services