A wordPress site will show 404 error message when someone try to visit a webpage but it doesn’t exist, or it has been removed, or its URL has been changed; or url is mistype. 404 error message actually show “not found” message.

Below are some very useful plugins to deal with 404 error pages:
1) 404 Notifier
This plugin will notify you of any 404 errors coming up on your site. It’s a very useful plugin to use if you’re changing your permalink structure or otherwise moving things around and are worried you may have missed some things and don’t want to risk broken URLs. This will help you catch those 404 error pages so you can take care of them.

2) Smart 404
This plugin will perform a search of your posts, pages, tags and categories, using keywords from the requested URL. If there’s a match, redirect to that content instead of showing the error. If there’s more than one match, the 404 template can use some template tags to provide a list of suggestions to the visitor. Thus Save your visitors from unhelpful 404 errors!

3) Google 404
The Google 404 widget is a quick and easy way to embed a search box on your custom 404 page and provide users with useful information designed to help them find the information they need. Where it can, the Google widget will also suggest other ways for the user to find the information they need, thus increasing the likelihood that they’ll continue to explore your site.

4) 404 SEO Plugin
Give yourself an SEO boost! This simple plugin will give you a customized, smart ‘Page Not Found(404)’ error message. It will automatically display links to relevant pages on your site, based on the words in the URL that was not found.

5) JH 404 Logger
JH 404 Logger is a very simple plugin that adds a Dashboard Widget showing recent 404 urls. The widget is easy to use and Ajax powered.

6) WP 404 images fix
Allows you to load a default alternative image where you are having image missing on your post. That is, if you embed an image in post, and the images is deleted or moved from the server, then you can load an alternative image to there by default. You can also add class to the img tag or just hide the image from front end users. You don’t have to open each the post and remove code from it.

7) No 404 Errors
This plugin changes 404 errors to 301 errors and redirects to a known page.