Freebbble!

To tell you the truth, I’m not quite sure what this is. Check it our yourself.

iGravertical Screen Layers + iOS 7 Screen Converter is a free UI design resource made by Dribbble user Balraj Chana. As far as I can tell, its license is Creative Commons Attribution.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.