Custom widget that displays value of a custom field in WordPress. http://sridharkatakam.com/create-custom-widget-displays-value-custom-field-wordpress/

plugin.php

// Widgets
include_once( BE_DIR . '/lib/widgets/widget-related-services.php' );

widget-related-services.php

<?php

/**
 * Adds SK_Related_Services_Widget widget.
 */
class SK_Related_Services_Widget extends WP_Widget {

	/**
	 * Register widget with WordPress.
	 */
	function __construct() {
		parent::__construct(
			'related_services', // Base ID
			__( 'Related Services', 'text_domain' ), // Name
			array( 'description' => __( 'Displays the content of Related Services custom field entered in individual Posts', 'text_domain' ), ) // Args
		);
	}

	/**
	 * Front-end display of widget.
	 *
	 * @see WP_Widget::widget()
	 *
	 * @param array $args     Widget arguments.
	 * @param array $instance Saved values from database.
	 */
	public function widget( $args, $instance ) {

		global $post;
	
		$related_services = get_post_meta( $post->ID, 'related_services', true );
		
		if ( empty( $related_services ) ) {
			return;
		}

		echo $args['before_widget'];
		if ( ! empty( $instance['title'] ) ) {
			echo $args['before_title'] . apply_filters( 'widget_title', $instance['title'] ). $args['after_title'];
		}
		// echo __( 'Hello, World!', 'text_domain' );
		echo $related_services;
		echo $args['after_widget'];
	}

	/**
	 * Back-end widget form.
	 *
	 * @see WP_Widget::form()
	 *
	 * @param array $instance Previously saved values from database.
	 */
	public function form( $instance ) {
		$title = ! empty( $instance['title'] ) ? $instance['title'] : __( 'Related Services', 'text_domain' );
		?>
		<p>
		<label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label> 
		<input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>">
		</p>
		<?php 
	}

	/**
	 * Sanitize widget form values as they are saved.
	 *
	 * @see WP_Widget::update()
	 *
	 * @param array $new_instance Values just sent to be saved.
	 * @param array $old_instance Previously saved values from database.
	 *
	 * @return array Updated safe values to be saved.
	 */
	public function update( $new_instance, $old_instance ) {
		$instance = array();
		$instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';

		return $instance;
	}

} // class SK_Related_Services_Widget

// register SK_Related_Services_Widget widget
function sk_register_related_services_widget() {
	register_widget( 'SK_Related_Services_Widget' );
}
add_action( 'widgets_init', 'sk_register_related_services_widget' );