How to create useful custom 404 page?

Okay, Google recommends us to have a custom 404 page. Not all sites that link to you will have correct incoming link. So they land on non-existent page. A custom 404 page will provide other helpful content and encourage visitors to explore your site further. How to create useful custom 404 page? I use AskApache Google 404 WordPress plugin to do the dirty work for me.

I customized it to display Google search results, related posts, recent posts and popular tags. Furthermore, it also implement noindex meta tag so that the 404 page won’t be indexed by search engine like Google and save PageRank juice for important pages. Isn’t that great?

Want to see an example? Click here to see it for yourself.