Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
Design for Humans - An email course about emotional design
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Simple Grid - Responsive & lightweight CSS grid for your website.
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.