I just noticed that a new feature is added into Google Webmaster Tools – Enhance 404 pages.
A 404 page will be delivered to the visitors when they tried to reach a non-existent resource on your site(broken link, unavailable URL, or even mistyped address). With a well designed 404 page, visitors could get more related info, such as knowing the reason why reach 404 page, finding other similar/related links within your site etc. In this way, visitors may spend more time on your site other than just surfing away.
With Google’s 404 widget, webmasters can easily implement a custom 404 page into their web sites. Just copy and paste the codes into your web pages where you want it to appear, the tool will automatically adds search box and other possible suggestions.
The good thing is, you can change what it looks like by modifying the CSS, which can make the 404 page fit to your site. And the tool is available in multi languages as well.
You should also be aware that this tool is still experimental, which means "we currently don’t provide closest match, alternative URL and sitemap suggestions for all sites."
Also, you may check out the custom 404 page guidelines by Google if you want to design yourself a 404 page.
