Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Bootstrap Zero - Open-source, free Bootstrap templates collection.
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.
HackerThemes - Bootstrap 4 themes and tools for web developers