Anima App - Design, get feedback, convert to code, publish, iterate.
Responsive Viewer - View multiple screen dimensions in one view
Auto-Layout for Sketch - Responsive design for Sketch
Responsive Image Breakpoint Generator - Turn one image into the ideal set of per-device images
Sketch Repo - Collection of resources for anyone who uses Sketch
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code