SELECT * FROM wp_options WHERE option_name = ‘active_plugins’;

global $wpdb;
$active_plugins = get_option(‘active_plugins’, FALSE);
$depends = ‘nextgen-gallery/nggallery.php’;
if ( ! in_array($depends, $active_plugins)) {
die(“Required plugin, {$depends} could not be found, please activate it before continuing”);
}

SELECT * FROM wp_options WHERE option_name = ‘active_plugins’;

A couple minor improvements to the usability: Instead of die(), just return. This will stop execution of the file, but won’t kill the entire wordpress stack.

To notify the user add an admin notice:

if (/*PLUGIN-NOT-INSTALLED*/) {
add_action(‘admin_notices’, ‘dependency_error’);
return;
}

function dependency_error () {
echo ‘Required dependency is missing.’;
}

in the case where a user has at one point installed NextGen Gallery (or in my case, WP e-Commerce Plugin), but gave up on it and uninstalled/deleted it, the db tables often might not get removed correctly, and your test would show that the plugin is there when it really isnt any more.