Facebook Design - Devices - Images and Sketch files of popular devices.
Anima App - Design, get feedback, convert to code, publish, iterate.
Design Cell - Beautiful multipurpose mockups for iOS
Sketch Repo - Collection of resources for anyone who uses Sketch
Device Shots - Generate high-resolution device mockups using a screenshot.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.