Banner Ad Under Posts

What I did was I modified my theme to add a custom hook under each post.

In my index.php for my theme, just before the <?php endwhile ?> quicktag, I added the following:

<?php do_action('end_post') ?>

You can then write a simple plugin to insert anything between the posts:

<?php
/*
Plugin Name: Banner Ad
Plugin Author: Jeremy Visser
*/

function jv_insert_banner() {
    ?><!-- do whatever banner stuff you want here --><?php
}

add_action('end_post', 'jv_insert_banner');

?>