Interfacer - Collection of more than 200+ free design resources
Mockup Generator - Create attractive product screenshots with just a few clicks
Facebook Design Resources - A collection of free resources made by designers at Facebook
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Freebiesbug - Collection of the best free web design resources.
Device Shots - Generate high-resolution device mockups using a screenshot.