Base UI Sketch Framework - Huge set of everything you need to design and prototype
Flinto - The prototyping tool designers have been waiting for.
SketchCasts - A weekly screencast, all about how to use Sketch
UiSketcher - Easily sketch user interface with your pen-enabled device!
Glyphy - Copy & paste fancy fonts + cool symbols ☞✺✐™
Remake - A way to make web apps with just HTML & CSS