Sketch Talk - A community for Sketch users of all skill levels
Flowkit - Sketch library for user flows/content maps/annotations
SketchCasts - A weekly screencast, all about how to use Sketch
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates
Sketch Basics - First official Sketch course on Treehouse
RookieUp Career Bootcamps - Tools to help designers build great portfolios and get jobs.