Device Shots - Generate high-resolution device mockups using a screenshot.
Facebook Design - Devices - Images and Sketch files of popular devices.
Smartmockups - Create stunning product mockups, easily and online.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Design Cell - Beautiful multipurpose mockups for iOS
Marvel (mobile) - Turn sketches into prototypes (new iOS & Android apps)