Google Webmaster Tools recently added an experimental new feature called Enhance 404 pages, to help webmasters simply create a custom 404 page by embedding a widget on their custom 404 pages. The widget suggests other URLs that exist that are relevant to what the user may be looking for. While Google Webmaster Tools says it is not always accurate as far as what website it suggests, I think it's better than not giving you any options of where to go, such as a default 404 page.
Here is what the widget would output on a page if implemented. It will suggest going back to the homepage, sitemap or searching for a specific query.
If you don't have
Google Webmaster Tools for your website, you should give it a try. It's free and you can view a lot of information to make sure the pages on your website are being crawled in Google. You can also submit a sitemap to help Google go through all the pages of your site for indexing as well.
