SketchCasts - A weekly screencast, all about how to use Sketch
Device Shots - Generate high-resolution device mockups using a screenshot.
Base UI Sketch Framework - Huge set of everything you need to design and prototype
Facebook Design - Devices - Images and Sketch files of popular devices.
Remake - A way to make web apps with just HTML & CSS
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.