Base UI Sketch Framework - Huge set of everything you need to design and prototype
Sketch Repo - Collection of resources for anyone who uses Sketch
UiSketcher - Easily sketch user interface with your pen-enabled device!
SketchApp.rocks - Index of all awesome things related to Sketch.
SketchCasts - A weekly screencast, all about how to use Sketch
Stash - Free Sketch resources, redefined.