Base UI Sketch Framework - Huge set of everything you need to design and prototype
Monsters for Sketch - Create your own monster character in seconds.
UiSketcher - Easily sketch user interface with your pen-enabled device!
Invision - Prototyping and collaboration for design teams
SketchCasts - A weekly screencast, all about how to use Sketch
Avataaars - Mix and match avatars with a Sketch library