The password for the non-registered user is generated via MD5 from the user's name. A returning customer will be able to place and order as long as he/she uses the same name and e-mail address. But someone with only the knowledge of the e-mail address will not be able to login. If the user decides to use the "Forgot your password" function he/she can login and get access to the usual functions for a registered user.
The module is free.
EDIT: Version 0.3. Removed ini_set('display_errors', 1) and error_reporting(E_ALL) from authentication.php.