This adds a custom column to show the feature image of a post.

Show Feature Image on Post Admin

<?php

/*
Plugin Name: Featured Image in Post List
Description: This plugin allows you to see the featured image for each post in the Post list.
Version: 0.1
License: GPL
Author: Chris Rouse
Author URI: www.chrisrouse.us
*/


  /*--------------------------------------------*
	 * Add the featured image to the Post list.
	 *---------------------------------------------*/

	/**
	 * Adds the image size for the thumbnails in the Post list.
	 * Currently set to 100x100 pixels.
	 **/

if ( function_exists( 'add_theme_support' ) ) {
  add_image_size( 'admin-thumb', 100, 100 );	}


	/*--------------------------------------------*
	 * Adds the new column to the Admin Post View as the first column.
	 *---------------------------------------------*/

add_filter( 'manage_edit-post_columns', 'my_columns_filter', 10, 1 );
function my_columns_filter( $columns ) {
 	$column_thumbnail = array( 'thumbnail' => 'Thumbnail' );
	$columns = array_slice( $columns, 0, 1, true ) + $column_thumbnail + array_slice( $columns, 1, NULL, true );
	return $columns;
}

add_action( 'manage_posts_custom_column', 'my_column_action', 10, 1 );
function my_column_action( $column ) {
	global $post;
	switch ( $column ) {
		case 'thumbnail':
			echo get_the_post_thumbnail( $post->ID, 'admin-thumb' );
			break;

	}
}

	/*--------------------------------------------*
	 * Sets the width of the Thumbnail column.
	 *---------------------------------------------*/

add_action('admin_head', 'my_column_width');

function my_column_width() {
    echo '<style type="text/css">';
    echo '.column-thumbnail { width:110px !important; overflow:hidden }';
    echo '</style>';
}



?>