Base UI Sketch Framework - Huge set of everything you need to design and prototype
Device Shots - Generate high-resolution device mockups using a screenshot.
UiSketcher - Easily sketch user interface with your pen-enabled device!
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
SketchCasts - A weekly screencast, all about how to use Sketch
Screen Size Map - An interactive map of screen sizes for responsive web design