This WordPress plugin by aliciagh enables search through all blogs into your WordPress Multisite by post title, post content or post author.
Multisite Global Search doesn’t work with single WordPress installation and it must be activated for all sites using “network activate” in the Administration Panel. This plugin is based on my other global search plugin, WPMU Global Search. It has some new features but if you want to keep using the older version of WordPress MU, you can install my other plugin.
Features
Multisite Global Search Widget. Show a search form in your sidebar.
Customizable results page URI.
Two different form types, vertical and horizontal.
Search across all network blogs or only in your blogs if you are logged.
Search results are showed in a page which contents the shortcode: [multisite_search_result]
Entries on every site across your installation appear in search results immediately after publication.
Receive results from your complete blog network, even sites you do not own or control.
Customizable style sheet for widget and results page.
Download and info: http://wordpress.org/extend/plugins/multisite-global-search/