This is a pre-installed action of Joomlaxi Forms(Free Extension), to take a look at Joomlaxi Forms click this link http://www.joomlaxi.com/support/documentation/item/joomla-registration-action.html
This action provides you the following ways by which a user can register on your site:
+ By email-id only.
+ By email-id and password.
+ By username, email and password
+ By name, username, email, password
+ By name, username, email, retype email, password, retype password