Flowkit - Sketch library for user flows/content maps/annotations
Anima App - Design, get feedback, convert to code, publish, iterate.
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates
Sketch Repo - Collection of resources for anyone who uses Sketch
Sketch Basics - First official Sketch course on Treehouse
Auto-Layout for Sketch - Responsive design for Sketch