LoginAs by Vince Ng

<?php
/*
Plugin Name: Login As...
Description: Allows site administrator to log in as any user by specifying their user login name
Version: 1.1
Author: Vince Ng
*/

add_action('admin_menu', 'addLoginAsMenu');

function addLoginAsMenu()
{
    if (is_site_admin())
    {
        global $error;

        if($_POST['user'])
        {
            require_once(ABSPATH . WPINC . '/pluggable.php');

            $userData = get_userdatabylogin($_POST['user']);

            if($userData)
            {
                wp_setcookie($userData->user_login, md5($userData->user_pass), true, '', '');
                wp_redirect(get_active_blog_for_user($userData->ID)->siteurl . "/wp-admin/");
                exit();
            }
            else
                $error = TRUE;
        }

        add_submenu_page('wpmu-admin.php', 'Login As...', 'Login As...', 10, 'loginAs', 'displayLoginAsPage');
    }
}

function displayLoginAsPage()
{
    global $error;

    if($error)
        print('<div id="message" class="updated fade"><p>' . $_POST['user'] . ' does not exist!</p></div>');
?>
    <div class="wrap">
        <h2>Login As...</h2>
        <p>Type in the user name to log in as that user.  You will be logged out and redirected to their dashboard.</p>
        <form action="" method="post">
            <input type="text" name="user" id="user" size="25" /><br />
            <input class="button" type="submit" name="submit" value="Login" />
        </form>
    </div>
<?php
}
?>