Flowkit - Sketch library for user flows/content maps/annotations
Alpe Audio - Audio courses to master new topics 'on the go'
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates
LearnUX.io - Video courses for UX/UI Designers
Sketch Basics - First official Sketch course on Treehouse
Piktab - Discover free graphic resources on every new tab