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.

Brad Parbs

surfboards

Quickly output a 404 for static files that aren’t found, rather than loading the normal 404 page.
Any static files ( images, text, pdfs, etc ) that don’t exist will 404 as soon as possible, rather than loading the entire WordPress application.
<h3>Details</h3>
By default, the list of extensions to check are the results of <code>wp_get_ext_types</code>, but can be filtered with <code>static_404_extensions</code>.
The output is a static page with the text <code>404 Not Found</code>, this text can be edited by filtering <code>static_404_message</code>.
A 404 status code will be used, but can be filtered with <code>static_404_response_code</code>.
Passing true to <code>static_404_should_process_request</code> will short-circuit and skip processing the request. This filter gets passed the current request.

Static 404

A WordPress plugin to quickly send a 404 for missing static files.

Dynamic 404 Page vs Static 404