Bootstrap Zero - Open-source, free Bootstrap templates collection.
Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
Cruip - Landing page templates for startups
Material Design for Bootstrap 4 - Powerful and free UI KIT for the newest Bootstrap 4 and Material Design
Bootstrap Studio - Powerful desktop app for creating responsive websites using the Bootstrap framework
Shuffle for Bootstrap - Bootstrap drag and drop builder for busy developers.