Add Google search capabilities to your site's 404 "page not found" template.
You should first set up a Google webmaster account and submit an XML sitemap of your site to ensure the Google 404 widget works effectively.
<?php if ( function_exists( 'google404' ) ) google404(); ?>
You could also add this code to your theme's index.php, single.php, image.php, archive.php and search.php below the HTML that is displayed if no posts are present (where you insert this may vary from theme to theme).
You may also use the
google_404_language filters for manually overriding the CSS values or language used by the Google 404 widget.