If you've been editing your WordPress theme in the admin area and you get the dreaded 'white screen of death' after saving, simply upload this to the root folder to select a working theme to reset to. You should then be able to access the admin ok to fix the bug in the offending theme.

reset-theme.php

<?php
 	define('SHORTINIT',true);
	
	$wp_dir = getcwd();
	// if WordPress isn't in the root (eg. set up for Git), do something like this:
	// $wp_dir = getcwd() . '/path/to/wordpress/subdirectory';
    
	require_once($wp_dir . '/wp-load.php');

	if (isset($_GET['to'])) {
		global $wpdb;
		$query = $wpdb->query('
			UPDATE
				wp_options
			SET
				option_value = "' . $_GET['to'] . '"
			WHERE
				FIND_IN_SET(option_name,"template,stylesheet,current_template")
		');
		echo '<p>The attempt to reset your WordPress template to <em>',$_GET['to'],'</em> was ',($query?'':'not '),'successful.</p>';
	} else {
		$dirs = array();
		$path = WP_CONTENT_DIR . '/themes/';
		foreach(scandir($path) as $item) {
			if (is_dir($path . $item) && !in_array($item,array('.','..'))) {
				$dirs[] = sprintf('<li><a href="?to=%1$s">%1$s</a></li>',$item);
			}
		}
		echo '<p>Found templates: </p><ul>',(count($dirs) ? implode('',$dirs) : '<li>No themes found.</li>'),'</ul>';
	}
?>