Bootstrap Zero - Open-source, free Bootstrap templates collection.
Anima App - Design, get feedback, convert to code, publish, iterate.
Bootstrap Magic - Create your Bootstrap 4.0 themes easily with magic
Sketch Repo - Collection of resources for anyone who uses Sketch
Material Design for Bootstrap 4 - Powerful and free UI KIT for the newest Bootstrap 4 and Material Design
Auto-Layout for Sketch - Responsive design for Sketch