Note: This feature is still experimental, and may not work well in 1.9. It doesn't work at all in 2.0 yet. Moodle is currently looking for a new maintainer to take this project on.
The Global Search, in Moodle 1.9 onwards, enables users to search all activity modules.
In addition to the 'Search Forums' and 'Search Courses', many users or designers might find the ability to search the actual content of the site/courses helpful. This can be done by adding the 'Global Search' block. It is not quite as simple as selecting the Global Search menu from the 'Add block' menu. If you do that- nothing happens. But do not despair. Simply perform the easy-to-follow steps of Global Search set-up (if your site is extremely large- do this at the end of the day to let the computer do the search overnight), but once the set up is complete, adding global search is as simple as clicking one button.