Allows you to add a Google 404 widget to your missing 'page not found' pages which allows you to do a Google search of the site.
A 404 page is what a user sees when they try to reach a non-existent page on your site (because they've clicked on a broken link, the page has been deleted or they've mistyped a URL).
While the standard 404 page can vary depending on your web host, it doesn't usually provide the user with much useful information and users may just surf away from your site. Therefore, Google recommend creating a custom 404 page that provides the user with more information about your site and its content.
The Google 404 widget is a quick and easy way to embed a search box on your custom 404 page and provide users with useful information designed to help them find the information they need. Where it can, the Google widget will also suggest other ways for the user to find the information they need, thus increasing the likelihood that they'll continue to explore your site.
You should set up a Google webmaster account and submit an XML sitemap of your blog to ensure the Google 404 widget works effectively.
Find out more here...
The Google 404 widget plugin is NOT a WordPress widget. It is simply the term Google use to describe their enhanced 404 page script.