A collection of free resources made by designers at Facebook
iOS 9 GUI (iPhone)
Photoshop and Sketch templates of GUI elements found in the public release of iOS 9
Devices
Images and Sketch files of popular devices
Diverse Device Hands
Photos of hands holding various phones, to be used in any presentation of your designs
Origami — Design Prototyping
A free tool for prototyping modern user interfaces. Get sample files and documentation to quickly build prototypes
Facebook Design on Medium
Experiences from the people designing products that billions of people use every day