Plugin T5 Excerpt Widget.

t5-excerpt-widget.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: T5 Excerpt Widget
 * Plugin URI:  https://gist.github.com/toscho/b318faa2b79d9bb8ffe8
 * Description: Shows the post excerpt in a widget. The widget is visible on single posts only.
 * Version:     17.02.15
 * Required:    4.0
 * Author:      Thomas Scholz <info@toscho.de>
 * Author URI:  http://toscho.de
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

class T5_Excerpt_Widget extends WP_Widget {

	public function __construct() {
		parent::__construct( 't5_excerpt_widget', __( 'Excerpt' ) );
	}

	public function form( $instance ) {
		$instance = wp_parse_args( $instance, array( 'title' => '' ) );

		print $this->get_title( $instance[ 'title' ] );
	}

	private function get_title( $title )
	{
		$title = esc_attr( $title );
		$id    = $this->get_field_id( 'title' );
		$name  = $this->get_field_name( 'title' );
		$label = __( 'Title:' );

		return "<p>
			<label for='$id'>$label
				<input class='widefat' id='$id' name='$name' value='$title' type='text'>
			</label>
		</p>";
	}

	public function update( $new_instance, $old_instance ) {

		$new_instance[ 'title' ] = esc_attr( $new_instance[ 'title' ] );

		return $new_instance;
	}

	public function widget( $args, $instance ) {

		if ( ! is_single() )
			return;

		$excerpt = get_post()->post_excerpt;

		if ( '' === $excerpt )
			return;

		$title = '';

		if ( ! empty ( $instance['title' ] ) )
			$title = $args['before_title'] . $instance['title'] . $args['after_title'];

		print $args['before_widget'] . $title;
		print wpautop( $excerpt );
		print $args['after_widget'];
	}

	public static function register() {
		register_widget( __CLASS__ );
	}
}

add_action( 'widgets_init', array ( 'T5_Excerpt_Widget', 'register' ) );