Bootstrap Zero - Open-source, free Bootstrap templates collection.
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Bootstrap Magic - Create your Bootstrap 4.0 themes easily with magic
Materialize CSS - A modern responsive front-end framework based on Material Design
Material Design for Bootstrap 4 - Powerful and free UI KIT for the newest Bootstrap 4 and Material Design
UIKit - A lightweight and modular front-end framework for developing fast and powerful web interfaces