aliciagh

Easily search through all blogs into your WordPress MU posts by post title, post content or post author. 
WPMU Global Search can manage multiple search forms. Currently in the following languages:
<ul>
<li>English</li>
<li>Spanish (es_ES)</li>
</ul>
If you have created your own language pack, or have an update of an existing one, you can send <a href="https://codex.wordpress.org/Translating_WordPress" rel="nofollow ugc">gettext .po and .mo files</a> to me so that I can bundle it into WPMU Global Search.

WPMU Global Search

Adds the ability to search through blogs into your WordPress MU installation.

Brad Williams

This plugin lets you create category mappings allowing for one top-level blog to act as a portal for your global community of WPMU blogs. Category postings are aggregated based on the mappings and displayed on the top-level blog.
Features:
<pre><code>* Map multiple top-level categories
* Built-in sidebar widget to display all categories
* WordPress MU 2.7 approved!
</code></pre>
<h3>Plugin Support</h3>
<a href="http://webdevstudios.com/support/wordpress-plugins/" title="WordPress Plugins and Support" rel="nofollow ugc">WordPress MU Category Mapping Plugin</a>

WordPress MU Category Mapping Plugin

Plugin links categories network wide to provide one top-level blog to act as a portal for your global community of blogs

WPMU Global Search vs WordPress MU Category Mapping Plugin