How to add custom admin columns in WordPress for a sample post type "szbl-location" – this is from my WordCamp San Diego 2013 presentation. This is a bit simplified.

admin-columns-hooks.php

<?php
/*
    Add the actual columns to the szbl-location edit listing admin screen.
*/
add_filter( 'manage_edit-szbl-location_columns', 'szbl_manage_edit_columns', 10 );
function szbl_manage_edit_columns( $cols )
{
    $new_cols = array(
		'cb' => '<input type="checkbox">',
		'title' => __( 'Location Name' ),
		'szbl-location-address' => __( 'Address' ),
		'szbl-location-phone' => __( 'Phone' ),
		'szbl-location-img' => __( 'Image' )
	);
	return $new_cols;
}

/*
Output custom column content.
*/
add_action( 'manage_szbl-location_posts_custom_column', 'szbl_manage_posts_column', 10 );
function szbl_manage_posts_column( $column_name )
{
    $post_id = get_the_ID();
    switch ( $column_name )
    {
        case 'szbl-location-address':
            $address get_post_meta( $post_id, 'szbl_location_address', true );
            echo esc_html( $address );
            break;
            
        case 'szbl-location-phone':
            $phone = get_post_meta( $post_id, 'szbl_location_phone', true );
            echo esc_html( $phone );
            break;
            
        case 'szbl-location-img':
            if ( has_post_thumbnail() )
            {
                echo '<a href="' . get_edit_post_link( $post_id ) . '">';
                the_post_thumbnail( 'thumbnail' );
                echo '</a>';
            }
            break;
    }
}