Controlled through the Widget menus. It adds sub-page navigation automatically

sub-page-navigation-widget.php

<?php

/*
Sub-Page Navigation Widget
Controlled through the Widget menus.
It adds sub-page navigation automatically.
It has no options.
*/

class SubPage_Navigation_Widget extends WP_Widget {

	// Constructor
		function SubPage_Navigation_Widget() {
			$sw_opts = array(
				'classname' => 'subpage_navigation_widget',
				'description' => 'Sub-navigation for pages that have child pages'
				);
			$control_opts = array( 'id_base' => 'subpage_navigation_widget' );
			$this->WP_Widget('subpage_navigation_widget', 'Page Sub-Navigation', $sw_opts, $control_opts );
		}


	// Outputting our Widget
		function widget($args, $instance) {
			extract( $args );
			
			// Set up our initial variables.
			global $post;
			$nav = '';
			
			// If we're not on a page that has a post (empty serch page?), let's just stop.
			if ( empty($post) )
				return;
			if ( is_category() || is_tag() || is_archive() || is_search() || is_home() || is_single() ) 
			{
				$title = 'Categories:';
				
				// Create a list of categories
				$args = array(
					'title_li' => '',
					'echo' => 0
				); 
				$nav = wp_list_categories( $args );
				
			} else {
				// Where are we?
				$parent_id = $this->get_parent_id($post);
				$title = get_the_title($parent_id);
				$link =  get_permalink($parent_id); 
				$title = "<a href='$link' title='$title'>$title:</a>";
				
				if (  !empty($parent_id)  ) {
					$args = array(
						'title_li' => '',
						'child_of' => $parent_id,
						'echo' => 0,
						'sort_column' => 'menu_order'
					);
					
				    $nav = wp_list_pages($args);
				}
			}
			
			// Let's hide this if there aren't any sub-pages or sibling pages... it tends to break.
			if ( !empty($nav) ) {
				// Before widget //
				echo $before_widget;
				
				// Output widget //
				if ( !empty($title) ) { echo $before_title . $title . $after_title; }
				echo "\n\t\t\t<ul id=\"sidebar-subnav\">\n";
				echo $nav;
				echo "\n\t\t\t</ul>";
				
				// After widget //
				echo $after_widget;
			}
		}


	// Update settings
		function update($new_instance, $old_instance) {
			$instance = $new_instance;
			return $instance;
		}


	// Widget Control Panel //

		function form( $instance ) {
			$defaults = array( );
			$instance = wp_parse_args( (array) $instance, $defaults );
			
			// Build the widget form
			?> 
				<p>No options for this widget. It will automatically build a navigation list based on the sub-pages of the top-level page.</p>
			<?php
		}
	
	// Helper to find the base ancestor of a page that's not the home page //
		function get_parent_id($post) {
			if ( $post->post_parent ) {
				$ancestors = get_post_ancestors($post->ID);
				$root = count($ancestors)-1;
				$parent = get_post($ancestors[$root]);
			} else {
				$parent = $post;
			}
			return $parent->ID;
		} // end get_parent_id
}
add_action('widgets_init', create_function('', 'return register_widget("SubPage_Navigation_Widget");'));