Anima App - Design, get feedback, convert to code, publish, iterate.
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!
Creatures - Mix and match your monsters and go wild with creativity
SketchCasts - A weekly screencast, all about how to use Sketch