Google widget for your custom 404 pages. The last entry in Webmaster Tools is called "Enhance 404 pages" and allows you to create custom 404 error pages. This widget can helps your visitors to find what they’re looking for by providing suggestions based on the incorrect URL.
Here is the example coming from Google webmaster blog:
Jamie receives the link www.example.com/activities/adventurecruise.html in an email message. Because of formatting due to a bad email client, the URL is truncated to www.example.com/activities/adventur. As a result it returns a 404 page. With the 404 widget added, however, she could instead see the first image.