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.

Yoast

Remkus de Vries

Redirectioner allows WordPress admins to have control over their dead links and redirects from inside the admin panel. <a href="https://redirectioner.com" rel="nofollow ugc">Redirectioner</a> records all URLs that users have visited and allows the admin to easily create 301 and 302 redirects to valid pages on their site. Redirects can also be created based on the best possible match for the URL the visitor was most likely trying to reach.
<blockquote>
Support & Bug Reports 
 If you’re in need of support or would like to file a bug report, please head over to our Github repository and <a href="https://github.com/thememix/redirectioner/issues" rel="nofollow ugc">create a new issue.</a>
</blockquote>
<h4>Features:</h4>
<ul>
<li>Get list of 404 URLs as they happen inside the admin panel</li>
<li>Easily redirect 404 URLs to existing pages or choose to ignore the 404 error</li>
<li>Provides the ability to automatically create redirects based on the URL the visitor was most likely trying to visit</li>
<li>Provide visitors with a list of suggested pages on the 404 page when a automatic redirect can not be made</li>
<li>Ability to suggest tag and category pages</li>
<li>Ability to create automatic redirect for misspelled tag and category pages</li>
<li>Ability to view logs of hits to 404 pages and redirects including referrer data</li>
<li>Ability to remove automatically remove redirects when the URL matches a new page or post permalink</li>
<li>Ability to automatically remove manual and automatic redirects once they are no longer being used</li>
<li>All features work with both pages and posts</li>
<li>Create automatic redirects for any URL resolving to a single page or post that isn’t the current permalink</li>
<li>Basic stats of plugin usage</li>
</ul>
Convert your 404 traffic by providing your site visitors with a better browsing experience and eliminate 404 URLs on your site.

Redirectioner

Creates automatic redirects for 404 traffic and page suggestions when matches are not found providing better service to your web visitors.

Dynamic 404 Page vs Redirectioner