Display a helpful 404 page. With links to posts/pages/CPT related to url keywords.

helpful-404.php

<?php
/*
  Plugin Name: Helpful 404 Page
  Plugin URI: http://clark-technet.com/
  Description: Displays helpful list of post/pages releated to the url on a 404 page.  Simply edit 404 page and use echo helpful_404().
  Author: Jeremy Clark
  Author URI: http://clark-technet.com
  Version: 1.0
  Author URI:
 */

function helpful_404() {
    global $wp_query;
    $output = "";
    //Removes extension from url
    $s = preg_replace( "/(.*)-(html|htm|php|asp|aspx)$/", "$1", $wp_query->query_vars['name'] );
    //Queries for post by name
    $posts = new WP_Query( 'post_type=any&name=' . $s );
    //If no posts are found use a search query
    if ( $posts->found_posts == 0 ) {
        //Removes dashes and underlines and replaces with spaces
        $s = str_replace( array( "-", "_" ), " ", $s );
        //Searches post and page content for string
        $posts = new WP_Query( 'post_type=any&s=' . $s );
    }
    if ( $posts->found_posts > 0 ) {
        $output = "<p>" . __('Maybe try one of the following pages?', 'theme_slug') . "</p>";
        $output .= "<ul>";
        while ( $posts->have_posts() ) {
            $posts->the_post();
            $output .= '<li><a href="' . get_permalink( ) . '">' . get_the_title() . '</a></li>';
        }
        $output .= "</ul>";
    }
    return $output;
}