Base UI Sketch Framework - Huge set of everything you need to design and prototype
Sketch Extensions - An official collection of Sketch extensions & plugins
SketchCasts - A weekly screencast, all about how to use Sketch
UiSketcher - Easily sketch user interface with your pen-enabled device!
SketchTool - Export pages & slices out of .sketch docs using CLI
Remake - A way to make web apps with just HTML & CSS