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.

LoudSkyMedia

This simple plugin permanently redirects all 404’s to the main blog URL. There may be several reasons why you might want to do this, but the primary purpose is to siphon Google Page Rank (TM) from missing pages to the homepage.

404 Redirection

Permanently redirect all 404's to the main blog URL. The primary purpose is to salvage Google PageRank (TM) from missing pages.

Dynamic 404 Page vs 404 Redirection