User modal module allows opening the Register/ Login/ Reset password menu items
as tabs. Since the tabs are shown via JS and not AJAX, this may lead to a better
user experience, as there is no time waiting for the selected tab to load.
The module requires the client have JS enabled.
The module comes with a well documented example module, that shows the user-modal and an example of how an implementing module can re-use the user-modal as subform to allow users to create an article node and login through a single modal window.
* Overlay (in Drupal core)
* Subform (Dev version).
Read more about it in this blog post