AskApache Google 404 by AskApache is a simple plugin that takes over the handling of any HTTP Errors that your blog has from time to time.
The most common type of error is when a page cannot be found, due to a bad link, mistyped URL, etc.. So this plugin uses some AJAX code, Google Search API’S, and a few tricks to display a very helpful and Search-Engine Optimized Error Page.
The default displays Google Search Results for images, news, blogs, videos, web, custom search engine, and your own site. It also searches for part of the requested filename that was not found, but it attaches your domain to the search for SEO and greater results.
This new version also adds related posts, recent posts, and integrates thickbox for instant previews.
Plugin homepage: http://www.askapache.com/seo/404-google-wordpress-plugin.html