Unfortunately, the default Recent Comments widget also includes trackbacks and pingbacks and the sight ain’t so lovely thus I had to add this hack to weed out the trackbacks and pingbacks.

Open your theme’s functions.php file and add the following code

if ( !$comments = wp_cache_get( 'recent_comments', 'widget' ) ) {
    $comments = $wpdb->get_results("SELECT $wpdb->comments.* FROM $wpdb->comments JOIN $wpdb->posts ON $wpdb->posts.ID = $wpdb->comments.comment_post_ID WHERE comment_approved = '1' AND post_status = 'publish' AND comment_type IN ('comment', '') ORDER BY comment_date_gmt DESC LIMIT 15");
    wp_cache_add( 'recent_comments', $comments, 'widget' );

Please note that the value red (DESC LIMIT 15) controls how many recent comments will appear in your sidebar and the order with which they will appear (ASC or DESC)