Base UI Sketch Framework - Huge set of everything you need to design and prototype
Design Systems Repo - A collection of design system examples and resources
UiSketcher - Easily sketch user interface with your pen-enabled device!
Lingo Sketch Plugin - A design system manager for Sketch
SketchCasts - A weekly screencast, all about how to use Sketch
Mobile Chat Kit - UI kits & sample apps for building in-app mobile chat