Base UI Sketch Framework - Huge set of everything you need to design and prototype
Select Similar Layers - Sketch plugin to select layers with the same attributes
INSTAD.IO - Automagically converts vector images into sketchy style
Framer Inventory 3 for Sketch - Import states & export prototypes from Sketch to Framer
Sketch Extensions - An official collection of Sketch extensions & plugins
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.