Flinto - The prototyping tool designers have been waiting for.
Sketch Talk - A community for Sketch users of all skill levels
Base UI Sketch Framework - Huge set of everything you need to design and prototype
SketchCasts - A weekly screencast, all about how to use Sketch
Glyphy - Copy & paste fancy fonts + cool symbols ☞✺✐™
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates