The Google Webmaster Central Blog announced the availability of a 404 Widget for websites, that would add more value to the error page shown, when a page does not exist on a website.

The widget adds a closest match URL based on the error page and the links available in the sitemap you upload to Google, along with adding a search box for searching related terms in Google.

In addition, you can add the 404 widget to your custom 404 page. The 404 widget automatically provides users with helpful suggestions instead of error messages.

• It adds a search box for your site with appropriate search suggestions.
• It tries to provide alternatives to incorrect URLs.

To get the code for the 404 page, visit your Google webmaster account and choose the Enhance 404 pages options from the Tools menu.

You would need to have a custom 404 error page in order to make use of this feature, you can find a tutorial on how you can add a custom 404 page to WordPress blogs or read a tutorial on adding it for a regular site.