Fabio Moro

4zero4 makes your 404 pages useful. When a visitor reaches a URL that no longer 
exists, the plugin builds a search query from the requested address and redirects 
the visitor to your site’s search results, so they land on relevant content 
instead of an empty error page.
For example, a request for <code>/my-old-product-name/</code> that no longer resolves is 
turned into a search for “my old product name”.
Key points:
<ul>
<li>Zero configuration. Activate it and it works.</li>
<li>Temporary redirect (302). The original URL may become valid again, so the 
redirect is never cached as permanently moved.</li>
<li>Front-end only. It never interferes with the admin area, feeds or the REST 
API, and only acts on normal GET page views.</li>
<li>No external services. The plugin does not send any data anywhere and does 
not use tracking or analytics.</li>
</ul>
<h4>Developer hooks</h4>
The behaviour can be customised with filters:
<ul>
<li><code>p4zero4_search_terms</code> — change (or clear, to skip) the terms extracted from the URL.</li>
<li><code>p4zero4_redirect_url</code> — change (or clear, to cancel) the destination URL.</li>
<li><code>p4zero4_redirect_status</code> — change the HTTP status code (default <code>302</code>).</li>
</ul>

4zero4

Turn "page not found" (404) errors into a helpful search instead of a dead end.

Matt Royal

This plugin dynamically creates pages for the most common error codes. It generates page templates for each error code from within your themes current page.php template file and assigns each new page template to the appropriate error page. It then updates your HTACCESS file with the correct references to these new pages.
Requires WordPress 3.0 +
Get in touch with me at: 
http://mattroyal.co.za/
Features
<ul>
<li>Dynamically creates all required pages</li>
<li>Creates page tempaltes and automatically asigns them to the newly created pages</li>
<li>Updates your HTACCESS file with the required commands to make the pages display when needed.</li>
</ul>
<h3>Usage</h3>
<ul>
<li>Enable the Plugin</li>
<li>Ensure your HTACCESS file and theme folder are writable</li>
<li>Check that pages have been created</li>
<li>Disable the plugin</li>
</ul>
Please Note – NB!!
<ul>
<li>In the event of moving your WordPress installtion into another directory, please note you will need to delete your HTACCESS file and resave it again.</li>
<li>Do not change the slugs/Permalinks of the newly created pages. If you do so you will have to update the new slugs within the HTACCESS file as well.</li>
</ul>

WordPress Easy Custom Error Pages

This plugin dynamically creates pages & page templates for the most common error codes/statuses and updates your HTACCESS file as required to work …

4zero4 vs WordPress Easy Custom Error Pages