Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
Anima App - Design, get feedback, convert to code, publish, iterate.
Bootstrap Zero - Open-source, free Bootstrap templates collection.
Sketch Repo - Collection of resources for anyone who uses Sketch
HackerThemes - Bootstrap 4 themes and tools for web developers
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.