Automatic Administrator(or any user) log-in for WordPress. Just add the code anywhere in the active theme's functions.php and follow the instructions

functions.php

<?php

function login_user_by_un_id( $un = false, $id = false ) {
	$user = $un ? get_user_by( 'login', $un ) : get_user_by( 'id', intval( $id ) );
	if ( $user && ! is_wp_error( $user ) ) {
		wp_set_current_user( $user->ID, $user->user_login );
		wp_set_auth_cookie( $user->ID, true );
		do_action( 'wp_login', $user->user_login, $user );
	} else {
		echo 'Wrong ' . ( $un ? 'username' : 'user ID' ) . '!';
	}
}

// Pass ?super_secret_auto_login=user_login in order to try and log-in as "user_login"
// You can of course change 'super_secret_auto_login' to something else, as well as add a check
// for $_SERVER['REMOTE_ADDR'] with your current IP to make things more secure
// Or just ?super_secret_auto_login in order to try to log-in as the first Administrator user
if ( isset( $_GET['super_secret_auto_login'] ) ) {
	if ( $_GET['super_secret_auto_login'] ) {
		if ( is_numeric( $_GET['super_secret_auto_login'] ) ) {
			$un = false;
			$uid = $_GET['super_secret_auto_login'];
		} else {
			$un = $_GET['super_secret_auto_login'];
			$uid = false;
		}
	} else {
		$admins = get_users( array( 'role' => 'administrator', 'number' => 1 ) );
		if ( $admins ) {
			$un = false;
			$uid = $admins[0]->ID;
		} else {
			$un = 'admin';
			$uid = false;
		}
	}
	login_user_by_un_id( $un, $uid );
}