Base UI Sketch Framework - Huge set of everything you need to design and prototype
Sketch Mirror - Preview your iOS designs directly on your devices
INSTAD.IO - Automagically converts vector images into sketchy style
Figma Mirror - Figma Mirror is an application that covers top trending designing or sketching ideas making your teamwork in a single environment and design better products from start to finish.
Sketch Extensions - An official collection of Sketch extensions & plugins
Flowkit - Sketch library for user flows/content maps/annotations