Base UI Sketch Framework - Huge set of everything you need to design and prototype
CSS Peeper - Smart CSS viewer tailored for Designers.
SketchCasts - A weekly screencast, all about how to use Sketch
Tailwind CSS - A utility-first CSS framework for rapidly building custom user interfaces.
UiSketcher - Easily sketch user interface with your pen-enabled device!
Purgecss - Easily remove unused CSS