This module provides a simple interface that allows registered users to use a login name which is different than their username.
Users can add, edit, or delete an alternate login name. Administrators can also assign an alternate login name when creating user accounts, and there is an option to allow users to select an alternate login name upon registration.
To use, simply enable the module, then visit the user edit page. Enter the alternate login name in the 'Alternate Login' textfield, and save.
Note that users can still login with their normal username--this just adds the option of another login name. Also note that an alternate login name may not be equivalent to any other current alternate login name, nor any current username.
Any user entity field, including uid and email, can be chosen as an alternate login, and will be checked for uniqueness. Also see Login Toboggan