Freebiesbug - Collection of the best free web design resources.
Device Shots - Generate high-resolution device mockups using a screenshot.
Designer Mill - Collection of Best Free Design Resources
Facebook Design - Devices - Images and Sketch files of popular devices.
Design Resources - A curated list of tools in one place to help designers
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.