Design Principles - An open source repository of design principles and methods
Device Shots - Generate high-resolution device mockups using a screenshot.
Facebook Design Resources - A collection of free resources made by designers at Facebook
Facebook Design - Devices - Images and Sketch files of popular devices.
Facebook - Connect with friends, family and other people you know. Share photos and videos, send messages and get updates.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.