Creates a widget area and displays it after every X number of posts in a Genesis theme.

widgetized_archive_loop.php

<?php
//* Do not copy the beginning <?php tag; place this code in your functions.php file

// Step 1: Modify the loop to accept an action hook
// Step 2: Create a widget area
// Step 3: Have the loop recognize the widget area
// Step 4: Place something in the new widget area to display
// Step 5: Style the area

/**
 * Step 1: Modify the loop to accept an action hook
 *
 * Display an ad after every 4th post on an archive page
 *
 * @author  Chris Mower
 * @package [Genesis Child Theme]
 */
add_action( 'genesis_after_entry', 'cm_ads_in_the_loop' );
function cm_ads_in_the_loop() {

    // make $loop_counter available to this function
    global $loop_counter;

    // start incrementing the counter
    $loop_counter++;

    // once the loop counter gets to four...
    if( $loop_counter == 4 ) {

        // and if it's on one of these page types...
        if( is_archive() || is_home() || is_search() ) {

            // then display whatever we want to hook into here.
            do_action( 'cm_in_the_loop' ); // yay, we have a new hook!
        }

        // set the counter back to zero
        $loop_counter = 0;
    }

}

/**
 * Step 2: Create a widget area
 *
 * This widget area will be used to display ads, but you can rename and 
 * use it for whatever your heart so desires!
 *
 * @author  Chris Mower
 * @package [Genesis Child Theme]
 */
genesis_register_sidebar( array(
    'id'          => 'loop-ads',
    'name'        => __( 'Loop Ads', 'cm-genesis' ),
    'description' => __( 'Use a \'Custom HTML\' widget and place your ad code within it. Recommended ad size: 728 x 90 Leaderboard.' ),
) );

/**
 * Step 3: Have the loop recognize a widget area
 *
 * Now that a sidebar is created, we're going to place it into 
 * the loop using our newly created hook. We're using Genesis 
 * markup so we have uniformity betwixt widget areas.
 *
 * @author  Chris Mower
 * @package [Genesis Child Theme]
 */
add_action( 'cm_in_the_loop', 'cm_place_widget_in_archives');
function cm_place_widget_in_archives() {
    // only display this if something is in the loop-ads sidebar
    if ( is_active_sidebar( 'loop-ads' ) ) {

        // Genesis markup for creating a widget area and tying it to
        // the sidebar we want
        genesis_widget_area( 'loop-ads', array(
            'before' => '<div class="loop-ads-widget-area"><div class="loop-ads widget-area">',
            'after'  => '</div></div>',
        ) );
    }
}

/**
 * Step 4: Place something in the new widget area to display
 *
 * No function here. Go to Appearance > Widgets and add something to
 * your new sidebar, then go check out your archive pages.
 *
 * @author  Chris Mower
 * @package [Genesis Child Theme]
 */

widgetized_archive_loop_styles.css

/** 
 * Don't forget to style your new widget area!
 * For starters, we'll just make sure it's centered.
 */

.loop-ads-widget-area {
    margin: 0 auto 40px;
}