Mockup Hunt is a huge collection of the best free mockups hand-curated from the most trusted mockup websites. Their collection is updated weekly.
Freepik Mockups is a platform where designers could find 100% free PSD mockups and Photoshop resources of the highest quality. And more…
Apple’s Developer Marketing Resources offer great mockups for all their devices, free to use when designers and developers want to market their work.
Facebook Design offers some of the best, high quality device mockups, UI kits, images and more. To be used by designers and developers for free.