The 404 or Not Found error message is a HTTP standard response code indicating that the client  was able to communicate with the server, but the server could not find what was requested.

Customizing error page 404 may be very important and useful, rather than letting visitor get default error page that say " 404 Page not found". Very likely the visitor will close and leave the pages of your website.

So why not give visitors something a little fun and interesting, with have a good navigation options or even redirecting them back to main page nor provide other important page and also search form for visitors to find content they want. Design your 404 error page should be in similar with the structure of your website so that visitors do not feel they have been expelled from the website, let they stay on your website.

In this examples design of "404 Error page" you can get inspiration and big smile. Take a look!

24-7 Media

CssTricks

Abduzeedo

Frye/Wiles

Henrik Hedegaard

Centerd

Cssscoop

Arbitraryconstant

Future of Web Design

Product Planner

Zivity

HomeStar Runner

d20 SRD

Dawdle

Icon Finder

Find Icons

Webexpedition18

Acromediainc

Jhuskisson

Psicobyte

Cgcraft

Just Creative

Bruno Bellamy

Blue Daniel

Surfingonstatic

Chrisjennings

Catalyststudios

Red Tag

Wufoo

Blizzard Entertainment

Popscreen

Carsonified

Sitesketch101

R Bardini

Dilbert

istockphoto

Reddit

Deviantart

Brightkite

Kidmondo

Btw, plus one, this is my 404-Error Page

And here some resources customize your wordpress 404 page;

• 8 WordPress Plugins Help Create Page 404 Friendly and Effective
• Customize your 404 page from the WordPress admin
• Creating an Error 404 Page