Allows you te set a page to be used as the 404 page. In order for the plugin to work you need to make some modifications to the <code>404.php</code> file in your theme. I set it this way so that the 404 page could be formatted to fit the theme, or can be set more dynamically.
For more information and sample code: http://www.reitzdesigns.com/2010/08/dynamic-404-page-wordpress-plugin/
Although this plugin will work with WordPress 2.8, it’s better suited for version 3.x.

Dynamic 404 Page

Allows you to select a page to use as the content of your 404 page.

George Pattichis

Link Juice Keeper redirects all non-existing URLs, that normally return a 404 error, to the page/post of your choice. This way everyone who comes to your blog via a broken link (both people and robots) will be redirected to a valid page.
A 404 Error (Page not Found) is very common and one of the main reasons Google can give you a lower ranking. Taking steps to handle 404 errors can be complex for non-techy users. With this free plugin you can finally resolve all 404 errors with just 1 click!
<h3>What are 404 errors?</h3>
As you probably know, incoming links play important role in ranking well in Google and other search engines. Therefore you should ensure that every incoming link leads to one of your valid pages on your blog. This may be a challenge because your website is dynamic and it changes every day:
<ul>
<li>you might remove or unpublish posts or pages;</li>
<li>you might change your URL structure on your blog;</li>
<li>links may brake because of technical misconfiguration;</li>
<li>a webmaster might add your link in their posts with a typo;</li>
<li>a visitor may simply mis-type your post’s URL</li>
</ul>
Any of the above will lead to link juice waste and a 404 error. You can avoid this simply by installing this plugin.
<h3>Features</h3>
<ul>
<li>Redirect errors to your homepage, an existing page, post or even a custom link</li>
<li>Monitor/log all 404 errors (Optional)</li>
<li>Choose redirect method to be used (301,302,307)</li>
<li>Send email notifications on every 404 (optional)</li>
<li>Translations ready</li>
</ul>
Link Juice Keeper was originally developed by <a href="https://profiles.wordpress.org/sirzooro/" rel="nofollow ugc">Daniel Frużyński</a>. The plugin has been adopted and refactored by <a href="https://profiles.wordpress.org/pattihis/" rel="nofollow ugc">George Pattichis</a> who will continue development and maintenance.
<a href="https://wordpress.org/extend/plugins/link-juice-keeper/changelog/" rel="ugc">Changelog</a>

Link Juice Keeper

Improve your SEO and keep your link juice by automatically redirecting all 404 errors to any page/post/url. User friendly options and log feature.

Dynamic 404 Page vs Link Juice Keeper