Base UI Sketch Framework - Huge set of everything you need to design and prototype
Frames for Sketch - Create awesome websites & apps using powerful design system
INSTAD.IO - Automagically converts vector images into sketchy style
Sketch Repo - Collection of resources for anyone who uses Sketch
Sketch Extensions - An official collection of Sketch extensions & plugins
Merge Duplicate Symbols - Automatically find and act on duplicate symbols in Sketch