I am looking to add to the create user feature to allow or display a drop down and or a text field that would allow the user to create a custom display name to associate with the post he or she would be creating.
While this doesn't allow for someone to choose their name per post, it does allow for someone to have a username and a "realname" http://drupal.org/project/realname
You could also just add a text field to nodes where they can fill out any name they want. Then plug that field into your theme instead of the username.
I don't know of any module etc. that would create this functionality.
This doesn't mean that it can't be done.
One way of doing this, would be to add a field on nodes, where the user could enter the author name. If present, you could then change the default author name with the value entered by the user.