Base UI Sketch Framework - Huge set of everything you need to design and prototype
Sketch Talk - A community for Sketch users of all skill levels
SketchCasts - A weekly screencast, all about how to use Sketch
UiSketcher - Easily sketch user interface with your pen-enabled device!
Sketch 101 by Designlab - Learn Sketch in this free course w/ downloadable templates
Remake - A way to make web apps with just HTML & CSS