Free Smartphone Mockups Freebie is a free Mockup design resource made by Dribbble user Daniel Ursuleanu. As far as I can tell, its license is WTFPL.
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.