Freebbble!

This one is the bomb, made by Teehan+Lax. Highly recommended. Actual license text:

The sole purpose of this file is to help you pitch, design and build amazing software. The file nor its elements are to be repurposed as your own. It can’t be broken apart and used to create similar tools.

iOS 7 UI Kit PSD Download is a free UI Kit design resource made by Dribbble user Teehan+Lax. As far as I can tell, its license is Free for Commercial and Personal Purposes.

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.