FlexboxPatterns - Build awesome user interfaces with CSS flexbox
Anima App - Design, get feedback, convert to code, publish, iterate.
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Fluid for Sketch - Sketch plugin to make your responsive designs more efficient
Git Sketch Plugin - Version control for designers
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.