Carrd - Simple, responsive one-page site creator.
Base UI Sketch Framework - Huge set of everything you need to design and prototype
Startup Framework - Create a beautiful, responsive website in under an 1 hour
INSTAD.IO - Automagically converts vector images into sketchy style
Front-End Checklist - The perfect Front-End Checklist for modern websites 🗂
Sketch Extensions - An official collection of Sketch extensions & plugins