Get list of installed Magento modules

modulelist.php

<?php

require 'app/Mage.php';
Mage::app();

$modules = Mage::getConfig()->getNode('modules')->children();

foreach ($modules as $moduleName => $moduleSettings) {
    echo '[' . ($moduleSettings->is('active') ? 'active' : 'disabled') . "]\t";
    echo $moduleName;
    echo "\n";
}