While my other login module does not have this feature, I decided to create a new login module with the popup function. Installation and setup of this popup login module is simple and easy. It works with Joomla 2.5 and 3.0 versions.
The module is not fancy, but it is doing exactly what it have to: press the login button and the login form will pop up in the center of the window marking the background black with 60% opacity.
If you have any question, I’m here to help!