login-relaxed.php

<?php
/*
Plugin Name: Login, Relaxed
Description: Logins to the admin dashboard are always redirected to `siteurl` by default. This plugin relaxes this to allow logins from URLs apart from `siteurl`. This is useful if you are logging in to WordPress from localhost.
Plugin URI: https://gist.github.com/rctay/6219624
Version: 0.1
Author: Tay Ray Chuan
*/

function wp_login_loginform_action_blank($url, $path, $orig_scheme=null)
{
	if ($path !== 'wp-login.php' || $orig_scheme !== 'login_post')
		return $url;
	return '';
}

add_filter('site_url', 'wp_login_loginform_action_blank', 10, 3);

?>