Alternative login ID & display names
This module allows users to log in using an alternate name instead of their username. It also provides options for administrators to set alternate login names and for users to choose one during registration. For Drupal 8, it can auto-populate usernames from various sources and permits login via user ID or email.
Drupal 7
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.
Drupal 8
Creates possibilities around Drupal's username and display name.
The username can be autopopulated from the user id, the address module's name fields, or from a callback of your choosing.
Use tokens to control the display name (install the token module for best effect!)
Finally it login is also permitted using the user ID or email fields. Also see Login Toboggan