How to add first, last and numbered classes automatically to widgets in WordPress. http://sridharkatakam.com/add-first-last-numbered-classes-automatically-widgets-wordpress/

functions.php

/**
 * Add "first" and "last" CSS classes to dynamic sidebar widgets. Also adds numeric index class for each widget (widget-1, widget-2, etc.)
 */
function widget_first_last_classes( $params ) {

	global $my_widget_num; // Global a counter array
	$this_id = $params[0]['id']; // Get the id for the current sidebar we're processing
	$arr_registered_widgets = wp_get_sidebars_widgets(); // Get an array of ALL registered widgets

	if( !$my_widget_num ) {// If the counter array doesn't exist, create it
		$my_widget_num = array();
	}

	// if( !isset( $arr_registered_widgets[$this_id] ) || !is_array( $arr_registered_widgets[$this_id] ) ) { // Check if the current sidebar has no widgets
		// return $params; // No widgets in this sidebar... bail early.
	// }

	if( isset( $my_widget_num[$this_id] ) ) { // See if the counter array has an entry for this sidebar
		$my_widget_num[$this_id] ++;
	} else { // If not, create it starting with 1
		$my_widget_num[$this_id] = 1;
	}

	$class = 'class="widget-' . $my_widget_num[$this_id] . ' '; // Add a widget number class for additional styling options

	if( $my_widget_num[$this_id] == 1 ) { // If this is the first widget
		$class .= 'widget-first ';
	} elseif( $my_widget_num[$this_id] == count( $arr_registered_widgets[$this_id] ) ) { // If this is the last widget
		$class .= 'widget-last ';
	}

	// $params[0]['before_widget'] = str_replace( 'class="', $class, $params[0]['before_widget'] ); // Insert our new classes into "before widget"
	$params[0]['before_widget'] = preg_replace('/class=\"/', "$class", $params[0]['before_widget'], 1); // Insert our new classes into "before widget"

	return $params;

}
add_filter( 'dynamic_sidebar_params', 'widget_first_last_classes' );