This module allows users to have a public name, shown as attribution and identity but not used for login. There are some significant requirements you must know about if you choose to use this module. See also http://drupal.org/project/realname for a module that accomplishes a similar purpose, but is more sophisticated and complex in its approach.
This module is different in approach than Realname (http://drupal.org/project/realname). With this module, the existing user name remains the value for the displayed name. This means that no additional code is needed for tokens, views, themes (for comment and post links, user picture, user page title, etc.), search results, etc. The result is a very small module that is also fool-proof against other contributed modules that may not be using theme('username', ...) to display the name of the user.
There is a lot of background about this issue, though mostly around the question of whether/how to include a public name feature in core. See:
You need to be using the email_registration and content_profile modules. You will need to set up Profile as a Content Profile and configure it appropriately. You will also want to assign appropriate permissions so that the appropriate users can create/edit own Profile content type.
The title of the Profile content type is linked to the user's 'name' attribute. The understanding is that you have chosen to use the content profile module for the same reasons we choose to use it. :-)
The strategy of this module requires using the email address to authenticate the user, instead of the user name attribute. This functionality is mostly provided by the email_registration module.
The user->name attribute is disabled for login use. Otherwise it works exactly the same. This means that all core and contrib functionality attributing, linking, listing users continues working the same way.
If you turn off the module after it has been in use, it should not hurt your system, but it will mean that users can log in with the 'public name' advertised as their identity on your site.
Enable email_registration and content_profile modules. Use the Profile content type or create/configure your own, to be the user's profile, the title of which will store the user's name.
The title of the configured Content Profile type (see below) is linked to the user's 'name' attribute.
In your content profile content type definition, on the content profile tab, you will find a checkbox that will set this content type to be the user name type. To avoid a highly interesting situation, you should only set this value to true for one content type!