Simple Registration simplifies the ExpressionEngine member registration process. You can output a registration form anywhere in your templates - so you don't need to edit the "member templates" to get a signup form (ie. the /member/register template).
You can also select which elements that are required in the signup form, and you get the form code generated so it's all plug'n'play.
Here's a video, note that it was made for the EE2 version of Simple Registration: