Anima App - Design, get feedback, convert to code, publish, iterate.
Sketch Talk - A community for Sketch users of all skill levels
Sketch Repo - Collection of resources for anyone who uses Sketch
SketchCasts - A weekly screencast, all about how to use Sketch
Auto-Layout for Sketch - Responsive design for Sketch
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates