Add a number to each widget in the sidebar.

Numbered WordPress widgets

function my_widget_class($params) {
global $widget_num;

// Widget class
$class = array();
$class[] = 'widget';

// Iterated class
$widget_num++;
$class[] = 'widget-' . $widget_num;

// Alt class
if ($widget_num % 2) :
$class[] = 'insert-even';
else :
$class[] = 'insert-odd';
endif;

// Join the classes in the array
$class = join(' ', $class);

// Interpolate the 'my_widget_class' placeholder
$params[0]['before_widget'] = str_replace('my_widget_class', $class, $params[0]['before_widget']);
return $params;
}
add_filter('dynamic_sidebar_params', 'my_widget_class')