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