Mockup Hunt is a huge collection of the best free mockups hand-curated from the most trusted mockup websites. Their collection is updated weekly.
Mockup`s Design is a simple site where designers can find premium-quality free mockups, to use in their projects.
Mr. Mockup offers free high quality Photoshop (PSD) mockups to showcase your design work like a pro!
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.
Apple Marketing Resources
Apple’s Developer Marketing Resources offer great mockups for all their devices, free to use when designers and developers want to market their work.