FlexboxPatterns - Build awesome user interfaces with CSS flexbox
Git Sketch Plugin - Version control for designers
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Fluid for Sketch - Sketch plugin to make your responsive designs more efficient
Sketch Plugin Course by Design+Code - Enhance your design workflow by creating a Sketch plugin
Anima App - Design, get feedback, convert to code, publish, iterate.