A loader for plugins that should run only in local stages. Have a local-plugins folder containing plugins into wp-content, and a local-config.php called from wp-config.php in your local stage. Then declare the relative path of your local plugins and load them.

local-plugins.php

<?php
/*
Plugin name: Local Plugins
Description: A loader for plugins that should run only in local stages.
Author: Andrés Villarreal
Author URI: http://about.me/andrezrv
Version: 1.0
License: GPL2
*/
function local_plugins_setup() {
	global $local_plugins;
	if ( isset( $local_plugins ) && is_dir( $local_plugins_dir = WP_CONTENT_DIR . '/local-plugins' ) ) {
		// Allow filtering for plugins list.
		$local_plugins = apply_filters( 'local_plugins', $local_plugins, $local_plugins_dir );
		foreach ( $local_plugins as $plugin ) {
			$file = $local_plugins_dir . '/' . $plugin;
			// Allow filtering for filepath.
			$file = apply_filters( 'local_plugin_filepath', $file, $plugin, $local_plugins_dir );
			if ( file_exists( $file ) ) {
				require( $file );
			}
		}
	}
	do_action( 'local_plugins_loaded' ); // Do stuff after loading the plugins.
}

if ( defined( 'WP_STAGE' ) && 'local' == WP_STAGE ) {
	$local_plugins = array(
		// The following values should be relative to WP_CONTENT_DIR . '/local-plugins'.
		'wordpress-beta-tester/wp-beta-tester.php',
		'wordpress-importer/wordpress-importer.php',
	);
	add_action( 'muplugins_loaded', 'local_plugins_setup' ); // Keep this line uncommented if you're using the plugin as MU.
	// add_action( 'plugins_loaded', 'local_plugins_setup' ); // Uncomment this line to use the plugin as a standard one.
}