On the administration you can find different search forms. Some for contents like posts, pages, media or comments. But you have also one for plugins, themes and/or users.
This plugin will centralize all search types into one search form, which is placed in the new Admin Bar. You can change the type via a dropdown menu.
The author says: “This plugin requires WordPress 3.3”
Download and info: http://wordpress.org/extend/plugins/admin-bar-backend-search/