Base UI Sketch Framework - Huge set of everything you need to design and prototype
Sketch Repo - Collection of resources for anyone who uses Sketch
SketchCasts - A weekly screencast, all about how to use Sketch
Charts by ls.graphics - The most comprehensive collection of charts for Sketch
UiSketcher - Easily sketch user interface with your pen-enabled device!
Gmail Add-on Design Kit - A simple Sketch library to speed up Gmail Add-on designs